Unsealing: Benefits, Potentials, Legal Provisions and Funding: The German Experience

Abstract

Soil ecosystem services are unavailable or very limited in urban areas with a high degree of soil sealing, thereby undermining the long-term quality of life of local residents at a time of climate change. To counter this, unsealing measures can contribute to soil protection and the provision of ecosystem services and thus to climate adaptation. In addition, unsealing measures are essential to compensate for new soil sealing and to ensure land degradation neutrality (SDG 15.3). However, the benefits of unsealing are undermined by the low availability of potential sites for unsealing and a number of obstacles to implementation.

In Germany, various legal instruments are available to activate unsealing potentials. Relevant regulations may be found in the fields of building and soil law as well as in water, nature protection and planning law. In practice, however, such regulations are limited in their impact. The paper shows how to better exploit unsealing potentials by a more consistent application of regulatory and planning legislation by authorities and legal revisions on the part of the legislator.

1 Introduction

To ensure sustainable development, it is essential to preserve natural soils and their ecosystem functions. Soils are key prerequisite for, among others, food and drinking water supply, but also assist in climate protection and the maintenance of biodiversity. In a nutshell: soils are crucial for the continued health and well-being of humankind. For instance, soils contribute to the infiltration and retention of rainwater, provide water to plants and facilitate evapotranspiration, which greatly helps to cool down the air and to improve the microclimate, especially in urban areas.

In the wake of urbanization, soil ecosystem services are destroyed or curtailed by soil sealing. Clearly, the unsealing of soils can greatly pave the way for the restoration natural soil functions and their associated ecosystem services. In this way, unsealed sites can help meet the goals of climate adaptation, particularly regarding water, soil and nature protection but also in safeguarding human health, for instance by reducing heat stress in cities or supporting flood prevention. Vegetation-covered restored soils may also contribute to climate protection by fixing carbon dioxide within plants and supporting the formation of soil organic matter (provided that there is adequate soil and plant management).

There is no doubt that soil unsealing can make an important contribution to the overarching goals of sustainable development such as land degradation neutrality (Sustainable Development Goal (SDG) 15.3¹) as well as the reactivation of ecosystem services to support climate adaptation and protection. Consequently, it is vital to implement measures for soil unsealing in order to restore soil functions and ensure better adaptation to the impacts of climate change. Hitherto, however, existing unsealing potentials have been insufficiently exploited to realize the mentioned goals.

In this article, we provide an overview of Germany’s experiences in soil unsealing. In Sect. 2 we present some definitions and goals for unsealing before outlining the benefits of unsealing (Sect. 3) and current potentials in Germany (Sect. 4). Then we discuss legal provisions for unsealing under German law (Sect. 5) and conclude with some key messages (Sect. 6).

2 Definitions and Goals of Unsealing

In this section, we define some important concepts around unsealing and outline its objectives.

2.1 Definitions

Soil is considered sealed if covered with impermeable layers such as asphalt, concrete, paving or buildings or if heavily compacted.² All (fully or partially) sealed areas with reduced or lost natural soil functions due to sealing are considered to be sites with unsealing potential. This includes plots which have been out of use for a longer period of time and/or will stay out of use in the foreseeable future, or plots whose current use can continue after a (partial) unsealing has taken place.

A complete unsealing of an area occurs when the physical barrier/covering layers, foreign materials and resulting soil compaction are completely removed and replaced by a soil type typical for the area with the aim of restoring as many of the natural soil functions as possible and/or creating a soil layer which can be inhabited by the root systems of vegetation. Partial unsealing refers to the incomplete removal of the profile layers of a sealed surface. Partial unsealing can take the form of:

• 1. unsealing of parts of a site,

• 2. changes in the artificial covering layer, or

• 3. functional unsealing (decoupling rainwater runoff from the sewer system).

After a partial unsealing measure, a considerable proportion of the original sealing material, substructure and/or anthropogenic influence remains present in the soil, thereby hindering the full restoration of all natural soil functions. However, a series of many small-sized partial unsealing measures located nearby, e.g. in backyards, may (especially in densely populated residential areas) also noticeably improve the local hydrologic balance or reduce heat stress. Partial unsealing is often more compatible when a site is subject to multiple forms of use, e.g. for walking or car parking. Of course, due to the specific features of each site (e.g. its slope, primary use or other practical issues), not every artificial covering is suitable to replace the original sealing layer. The decoupling of rainwater runoff from the sewer system may be realised, for instance, by storing the rainwater in an infiltration trench or pond. In general, partial unsealing aims to mitigate the impact of soil sealing, whereas complete unsealing provides compensation opportunities for new soil sealing undertaken elsewhere.³

2.2 Goals of Unsealing

While not an end in itself, unsealing helps smooth the way towards achieving the following three goals for sustainable development:

• 1. Restoration of soil functions and enhancement of ecosystem services: After unsealing, the uncovered soils can be reactivated and physical, chemical and/or biological functionality of the soils may be restored. This may enhance the supply of ecosystem services. If appropriate soil and plant management is ensured, the resulting carbon dioxide fixation by vegetation and the formation of soil organic matter can also assist in meeting climate protection goals.

• 2. Climate adaptation: The restored ecosystem services following unsealing may contribute to climate adaptation goals, particularly those related to health, water, soil and nature protection, provided that an appropriate concept for the usage of the unsealed area is implemented, for instance as a tiny forest,⁴ a pocket park⁵ or a retention area.⁶

• 3. Land degradation neutrality (SDG 15.3): As soil sealing is the most common cause of land degradation in Germany,⁷ soil sealing may be reduced or even halted at a certain level in order to maintain or improve the quantity and quality of land resources as a precondition for sustainable development,⁸ for instance to maintain food security (SDG 2) or the supply of clean drinking water (SDG 6). Alongside measures to avoid and reduce new land degradation, the goal of land degradation neutrality also demands that degraded soils be restored (for instance in the wake of unsealing).⁹ Unsealing of soils may serve as compensatory measures for newly sealed areas.

3 Benefits of Unsealing

The benefits of unsealing measures comprise the improved provision of ecosystem services and the resulting contributions to climate adaptation and sustainable urban development.

3.1 Provision of Ecosystem Services

After soil sealing, no interaction can take place between the pedosphere and the atmosphere or biosphere (which equates to the ecosphere), thereby greatly inhibiting exchange processes such as infiltration and evaporation, gas exchange and biotic processes. In this way, the natural soil functions as defined in Section 2 para. 2 no. 1 of the German Soil Protection Act (BBodSchG 1998) are impaired.

Soil sealing may result in the removal of humus and an input of translocated substrates, soil compaction and changes to the soil profile and land relief. Such impacts can also affect the soil functionality after unsealing and restoration, as they are only partly reversible. For this reason, it is essential to prevent the sealing of pristine, well-functioning soils. Although unsealing and recultivation measures are expensive, associated with high opportunity costs and may not completely restore soil functionality, they nonetheless contribute to the provision of relevant ecosystem services, especially in urban areas with limited supply of ecosystem services.

Regarding restoration measures, the reestablishment of physical soil functions such as infiltration and storage of rainwater, groundwater recharge and evapotranspiration requires the least effort. The reactivation of these processes may positively impact the local hydrological balance, leading to higher soil moistures and evaporation from soils, which also bring some cooling and positive microclimatic effects. By contrast, much more effort is required to restore the chemical soil functions such as nutrient transformation and storage processes as well as the biological soil functions such as decomposition of mulch, soil respiration and mineralisation caused by the biocoenosis of the soil. Yet, chemical and biological soil functions enable long-term stable and self-sustaining habitats, e.g. city forests. In general, the climate protection function of soils arises through the interplay of biological and chemical soil functions, whereby biomass is decayed and fixed in the soil as organic matter and humus.

3.2 Contribution to Climate Adaptation

Clearly, the impacts of climate change such as higher risks of flooding and heat stress are intensified in sealed areas. Soil unsealing can usefully restore ecosystem services and thereby contribute to climate adaptation, e.g. by increasing evaporative cooling (for an overview see Fig. 1). Central to the climate adaptation goals mentioned in this article are the rainwater retention function of soils and groundwater recharge. In many cases, synergy effects can be exploited to achieve different climate adaptation goals.¹⁰ For example, a water-sensitive urban development can recharge groundwater through rainwater infiltration and retention while at the same time reducing both, surface runoff and the likelihood of drought (water protection-related climate adaptation goals). Cooling via evapotranspiration may reduce the intensity of the urban heat island effect, the number of tropical nights and/or the annual hours of heat stress¹¹ (health protection-related climate adaptation goals). Furthermore, a smart design and management of neighbourhoods to ensure a greater volume of urban vegetation can improve both, the attractiveness of the residential environment and the well-being of local people.¹² Simultaneously, the provision of habitats for flora and fauna will improve biodiversity and contribute to the achievement of nature protection-related climate adaptation goals. Additionally, greater rainwater infiltration may reduce surface runoff peaks and thus, may reduce the risk of erosion in adjacent areas (soil protection-related climate adaptation goals).

Fig. 1

Intensification of climate change impacts in sealed areas (A–H) and potential contributions of soil functions in unsealed and natural soils to climate adaptation (1–6). (A) heat stress, (B) changes in biodiversity, (C) changes in soil development, (D) erosion, (E) changes in groundwater level, (F) low surface water levels, (G) soil drought and low air humidity, (H) floods; (1) rainwater infiltration, (2) rainwater retention and reduction of surface runoff, (3) evaporative cooling, (4) fresh air production from green open areas, (5) shade from trees and shrubs, (6) increased biodiversity and plant productivity (Own illustration based on Pannicke-Prochnow et al. (2021), p. 85)

Regarding the water protection-related climate adaptation goals, partial and complete unsealing measures may contribute significantly to the realisation of Sustainable Urban Drainage Systems (SUDS) such as in Shanghai, China,¹³ Water Sensitive Urban Designs (WSUDs) such as in Australia¹⁴ and Great Britain¹⁵ or “Schwammstadt” (“sponge city”) concepts found in Germany.¹⁶ These climate adaptation concepts make use of many scattered, spatially distributed, larger and smaller measures combined with a certain flexibility in application and the exploitation of synergy effects. Unsealing measures and the restoration of soil functions (in connection with appropriate land use concepts) are particularly suited to accomplish climate adaptation goals as they provide a number of synergies and are widely applicable despite the huge range of differences, opportunities and priorities between sites.¹⁷

3.3 Contribution to Sustainable Urban Development

Unsealing measures in urban areas may provide additional co-benefits for other municipal development goals, such as increasing the extent of green spaces for recreation, leisure, sports and social interaction, thereby improving the quality of local recreation and the resilience of urban residents by reducing their social and psychological stress.¹⁸ Especially in structurally disadvantaged areas, the creation of open areas as meeting points, playgrounds and sports fields may be key measures for upvaluation.¹⁹ The motivation of local residents to use these areas is largely determined by spatial proximity.²⁰ The lockdowns imposed in 2020 and 2021 by the coronavirus pandemic highlighted the importance of green and open space provision within each neighbourhood, and was a timely reminder to municipalities of their responsibility to provide such spaces to ensure a good quality of life.

Furthermore, unsealed and renaturated areas may improve the general health and well-being of urban residents by boosting the formation and supply of fresh air, enhancing the local bioclimate (which can reduce the likelihood of stress-related health problems such as heart attacks) and reducing particulate matter in the air.²¹ Furthermore, protective measures on unsealed areas, for instance against flooding, may be supported by the retention of rainwater in the restored soil. This helps lower the risks to public safety as well as potential costs for repairing damaged buildings and infrastructure. In addition, concepts for the multifunctional usage of sites may provide benefits for a range of actors. For instance, a park or city forest can be used for recreation, as a retreat or for nature observation or for sports; a short-rotation coppice may link regional value creation to open space design, noise protection, nature and soil protection; an urban garden may serve as a meeting and educational point as well as a source of regional food.

The challenge for sustainable urban development is to maintain a balance between re-densification and the preservation of a sufficient supply of attractive green and open spaces. An important prerequisite for the future viability of settlement and living areas is to ensure a good quality of life within a comparatively cost-efficient climate adaptation path.

4 Potentials for Unsealing in Germany

Generally speaking, the availability of unsealing potentials in Germany is mainly limited by technical, economic and political factors creating different levels of unsealing potential. Specifically, we can say that: (1) The theoretical potential for unsealing comprises all those areas which are not going to be used in the foreseeable future or where changes in use enable a partial or complete unsealing. (2) This potential is decreased to a technically feasible potential determined by the current technical options for soil unsealing and restoration, also taking into account possible restrictions due to contamination or architectural features, etc. (3) This is further decreased to an economically feasible potential determined by the costs of unsealing and restoration measures while considering all the available implementation and usage options as well as realisation, follow-up and opportunity costs. (4) Finally, this potential may be reduced to a politically feasible potential in consideration of the political and social (majority) consent needed to approve unsealing measures, especially against the background of conflicting urban development goals, e.g. more housing vs. open green spaces for maintaining quality of life. This aspect is particularly challenging due to high opportunity costs for municipal authorities facing low municipal budgets.

Economic and political feasibility are the two main obstacles to the implementation of unsealing measures. Typical problems arising in this regard are limited financial capacities, inappropriate legal guidelines on the obligations of municipalities and private landowners, conflicts in land use, a lack of political majorities and poor awareness of the contributions that unsealing can make to climate adaptation and other municipal development goals. And, of course, political decisions may significantly alter the economic, financial and capacity-related framework conditions.

In Germany, the theoretical supply of potential unsealing sites is rather limited and unevenly distributed geographically. A theoretical renaturation potential exists for approximately 1% of the area for buildings and their associated open spaces, even though only about half of this area is actually sealed. A further 1% of the area for buildings and their associated open spaces is considered as building land for short-term use and another 3–5% as reserve sites for inner development.²² Additional unsealing potentials may be available in the form of small-scale sites in residential areas for partial unsealing measures as well as in traffic areas, especially in car parking spaces, for partial and complete unsealing measures. These potentials may rise in future years due to shifting patterns of mobility.

5 Legal Provisions for Unsealing

Germany’s body of environmental and planning law provides several mechanisms that require or at least promote the unsealing of soil. In the following, we analyse these instruments with regard to their prerequisites, substantive scope and significance in practical enforcement before making some practical proposals for refining the legal framework. The results draw on an analysis of relevant literature and case law as well as interviews with experts.

5.1 Unsealing Obligations in Building and Soil Protection Law

The obligation to implement unsealing measures is primarily regulated by Section 179 of the Federal Building Code (BauGB 2017) and Section 5 of the Federal Soil Protection Act (BBodSchG 1998). However, these are rarely if ever applied in practice.

5.1.1 Section 179 BauGB

Section 179 para. 1 BauGB stipulates that, under certain conditions, a municipality can oblige a property owner to tolerate the complete or partial removal of a building. Such work will often necessitate soil unsealing. Para. 1 distinguishes between three circumstances that trigger such an obligation to tolerate: firstly, if the building is in conflict with the designations of the so-called “binding land-use plan” (sent. 1 no. 1); secondly, if the building displays irremediable defects and deficiencies that cannot be rectified (sent. 1 no. 2); and thirdly, if the goal is to rehabilitate a permanently unused area (sent. 2). As in the case of sentence 1 no. 1 (and unlike no. 2), the third variant requires that the building conflicts with the designations of the development plan. This is an “ecological variant” of the demolition requirement under sent. 1 no. 1, which is intended to emphasise the special importance of environmental context.²³

In practice, the demolition and unsealing requirements of Section 179 BauGB are rarely applied.²⁴ The few examples that we could identify relate exclusively to cases under Section 179 para. 1 no. 2, i.e. demolition in the case of unsatisfactory urban conditions and defects. The patchy application of these legal instruments can be attributed to a lack of experience on the part of the authorities as well as a reluctance to engage in legal dispute and possibly face litigation. And, of course, the municipality has to bear the costs of unsealing; the owner is only required to “tolerate” the measure.²⁵ The only cases where Section 179 para. 1 no. 2 BauGB has been applied to demolish dilapidated buildings and unseal areas are when municipalities could use earmarked budgets to this end.

5.1.2 Section 5 BBodSchG

Section 5 BBodSchG is a further legal instrument that can be used to unseal soil. This, however, may only be applied subsidiarily to building law (Section 3 para. 1 no. 9 BBodSchG). Section 5 sentence 1 BBodSchG authorises the federal government to adopt a legal ordinance obliging the landowner to unseal soil under certain conditions. In such cases, the land must be permanently unused and its sealing must conflict with planning regulations. The obligation relates to restoring the functional capacity of the soil as far as is reasonable and possible. In the absence of an ordinance under Section 5 sentence 1, the competent authority may also issue an individual unsealing order against the owner based on section 5 sentence 2, provided that the requirements of sentence 1 are met.

In contrast to Section 179 BauGB, which has been applied in several cases, Section 5 BBodSchG has never been enforced.²⁶ The federal government has neither issued an ordinance on unsealing pursuant to sentence 1 nor have the soil protection authorities ever made use of the individual authorisation in sentence 2, although some efforts have been undertaken in the past to make the regulation easier to enforce. For example, within the framework of a research project commissioned by the Federal Environment Agency, an unsealing ordinance was drafted in accordance with Section 5 BBodSchG, the feasibility of which was tested in a simulation game in 2005.²⁷ However, the simulation game proved rather unproductive as the participants were generally critical of the regulatory approach of Section 5 BBodSchG.²⁸

This criticism included the lack of prerequisites under planning law, the expense of large-scale sealing in outdoor areas, a lack of obligations to cooperate, weak official powers of investigation as well as poor financial incentives for unsealing. All of these factors are needed to ensure a certain willingness to cooperate and thus help enforce the ordinance.²⁹ The subsidiarity of the regulation vis-à-vis building law and its interpretation also adds to the confusion. Moreover, the relevant authorities have not yet developed any practical system for the regulatory enforcement of unsealing.

5.1.3 Proposed Amendments

The analysis of Section 179 BauGB and Section 5 BBodSchG shows that their areas of application must be clearly demarcated. In this regard, we propose that Section 179 BauGB should apply to sealing in inner settlement areas (i.e. the responsibility of the municipality) and Section 5 BBodSchG should apply to sealing in outlying areas (i.e. the responsibility of the soil protection authority).³⁰

Further, the scope of Section 179 BauGB should be extended to the entire inner area of a municipality to increase the effectiveness of the regulation. In addition, municipalities should be generally authorised to order unsealing measures to restore natural soil functions, not just when a building conflicts with a development plan or in the case of unsatisfactory conditions and deficiencies. In addition, the owner’s duty to “tolerate” unsealing measures by the municipality (cf. Section 179 BauGB) should be converted into an active duty to act.³¹ It is difficult to justify the current unequal treatment of property owners depending on whether they are obliged to unseal under building law (Section 179 BauGB) or under soil protection law (Section 5 BBodSchG).

In addition, we recommend that Section 5 BBodSchG be simplified: not only should the general primacy of building law be dropped, but also the requirement that the Federal Government be the relevant authority to issue an unsealing ordinance.³² Instead, the regulation should be formulated in such a way that it can be enforced on its own. The requirement of contradiction with planning regulations (cf. Section 5 BBodSchG) also does not appear necessary, as the norm could be undermined in the absence of concrete plans. Finally, the proportionality of the official order to unseal should be specified by a provision on the bearing of costs.

With regard to both Section 179 BauGB and Section 5 BBodSchG, we recommend that public bodies be made more responsible for unsealing measures. In this respect, regulations should require that disused, publicly-owned structural facilities should in general be unsealed.³³ For example, Section 179 BauGB could include an obligation for municipalities to set a good example in the unsealing of public spaces, properties and municipal (transport) infrastructure that are no longer needed.

However, the further development of the legal basis alone is unlikely to be sufficient to ensure the enforcement of unsealing orders. For the implementation of Section 179 BauGB and Section 5 BBodSchG, it is vital that concrete responsibilities be designated within the authorities and that municipal employees be supported at the working level by formulating procedural standards for the application of regulations. This can be done, for example, through further training, the provision of working aids, sample certificates, flow charts and standard procedures as well as internal and inter-communal exchange of experience.³⁴

At the same time, the obligations to unseal should be supplemented with advice for property owners and the provision of subsidies. Interviews with practitioners have confirmed that financial incentives are the strongest driver for politically desired changes.³⁵ Section 175 BauGB explicitly provides for consultation and the consideration of grants from public funds when issuing urban development orders. Financial support from the public purse may also be needed to ensure the proportionality of the measure while respecting the property rights of the owners.

5.2 Unsealing Obligations in the Undesignated Outlying Area

Section 35 para. 5 sentence 2 BauGB stipulates a special deconstruction (unsealing) obligation for facilities in the undesignated outlying area after permanent abandonment of the permissible use. These are, for example, buildings (and sealing) for market gardens, for the public supply of electricity, gas, telecommunications, heat and water, and for the use of wind and water energy (e.g. wind farms) or biogas plants.

The expert interviews conducted for the study showed that this legal provision is indeed applied and considered effective: the obligation is stipulated in connection with the granting of the building permit and thus can be secured.³⁶ However, physical structures in the undesignated outlying areas are exempt from the obligation to deconstruct if they serve some agricultural or forestry activity and only occupy a minor proportion of the total plot (Section 35 para. 1 no. 1 BauGB). This exemption should be removed, as there is in fact no real justification for it.

5.3 Urban Redevelopment and Urban Restructuring

Urban redevelopment measures (Sections 136 ff. BauGB) and urban restructuring measures (Sections 171 ff. BauGB) are area-based instruments that initiate unsealing measures in built-up neighbourhoods.

5.3.1 Urban Redevelopment

Urban redevelopment aims to substantially improve or redesign an area that shows some deficit. Such measures must be initiated by a public authority. Sections 136 ff. BauGB provides a suitable framework for this.

An area can be formally designated as a redevelopment area if the results of preparatory investigations by the municipality reveal urban deficits (Section 142 para. 1 BauGB). Such deficits are when the area does not meet the basic requirements for healthy living and working conditions or for general safety. This can also explicitly be the case with regard to climate-related concerns (i.e. climate protection and adaptation, Section 136 para. 2 no. 1 BauGB).

Section 147 BauGB lists the redevelopment measures that the municipality must implement to comply with the redevelopment statutes. These include unsealing activities such as the complete or partial removal of structures, the clearing of fill or road surfaces, the clearing of storage areas, the removal of soil sealing and the removal of contamination.³⁷ Urban redevelopment is normally associated with rather small-scale measures such as the unsealing of backyards.

5.3.2 Urban Restructuring

In contrast to urban redevelopment, urban restructuring (Section 171a BauGB) is an instrument for conceptualising and implementing large-scale unsealing measures. It serves to create sustainable urban structures where there is a considerable loss of urban function. The latter is particularly the case if there is a current or projected long-term oversupply of buildings for certain uses, namely for residential purposes. In practice, urban redevelopment is usually associated with the demolition of prefabricated buildings to eliminate vacant housing, the removal of industrial wastelands or former barracks.³⁸

One acute reason for urban restructuring is when entire residential neighbourhoods or housing estates have become vacant. However, this is not only about the demolition of individual buildings, but also an area-related conceptual development that should take into account the needs of the residents as well as urban development concerns, including environmental protection and climate adaptation, in a sustainable manner.³⁹ The basis for restructuring activities is an urban development concept drawn up by the municipality. Basically, we can say that the regulations on urban restructuring offer considerable potential for implementing unsealing measures.

5.3.3 Implementation and Realisation in Practice

In order to implement the planned urban redevelopment and urban restructuring measures, the municipality can negotiate mutually agreeable arrangements and sign contracts with the property owners concerned (Section 146 para. 3 and Section 171c BauGB). If amicable solutions cannot be found, the municipality can resort to urban development regulations (cf. Section 175 ff. BauGB). In particular, the owner can be obliged to tolerate the removal of a building by means of a demolition order pursuant to Section 179 BauGB.⁴⁰ However, the removal of structures can also take place after acquisition or expropriation by the municipality. In practice, financial incentives are mainly provided in the form of urban development grants (cf. Section 164a, 164b BauGB).

From the numerous good examples of urban redevelopment and restructuring cited by the interviewed experts, it can be concluded that these legal instruments function well.⁴¹ In general, it can be stated that even in urban redevelopment areas, no demolition orders are issued in accordance with Section 179 BauGB; instead, unsealing measures are taken voluntarily on the basis of discussions and consultations with the owners, often in combination with financial incentives. If the funding agency requires the municipality to issue a demolition order in accordance with Section 179 BauGB as a prerequisite for funding, the deconstruction order is issued pro forma.⁴²

5.4 Urban Land Use Planning

Unsealing measures can be conceptually prepared by means of urban land use planning. This is the case when existing urban districts are being redesignated. Thus, in addition to areas for building development, areas for unsealing and renaturation can be provided for in the urban land use plans (cf. Sections 5 and 9 BauGB). In principle, the redesignation of land can even be carried out with the exclusive objective of improving the environmental situation in the planning area.⁴³ This is guided by the requirements formulated in Section 1 and 1a BauGB, which include soil protection and climate adaptation.

However, the protection of existing buildings under building law sets limits to redesignation. For example, the designation of green spaces cannot interfere with existing buildings. If existing buildings are to be demolished, e.g. in order to secure areas needed for the establishment of a public green space by the municipality, this would only be possible by way of expropriation (cf. Section 85 BauGB). However, expropriation is a measure of last resort, for which owners have to be compensated.⁴⁴

Inner-city brownfield sites represent an important potential for unsealing. Generally, such brownfield sites should not be designated in development plans solely for the construction of buildings but also for the creation of open spaces and green areas close to residential areas. This idea is expressed in the guiding principle of “double inner development”.⁴⁵ The planning principle of inner urban development designated in the Federal Building Code (cf. Section 1, 1a and 176a BauGB) should be understood in this dual sense.

5.5 Building Regulations of the States

The building laws of the federal states (Länder) can also mandate unsealing measures.

5.5.1 Removal Orders for Reasons of Illegality, Hazard Prevention or Forfeiture

The building laws include the obligation to remove buildings which are illegally constructed (e.g. Section 80 Saxon Building Code – SächsBO 2016). Such demolition orders are the last resort against unauthorised physical structures that are formally and materially in breach of building law.⁴⁶

On the basis of hazard prevention, deconstruction orders can also be issued to avert dangers emanating from dilapidated properties.⁴⁷ It should be borne in mind, however, that any danger to life and limb is likely to be eliminated if a structure has been physically secured or demolished down to the height of the foundation walls. Generally, therefore, complete unsealing with restoration of the soil functions is not required from the perspective of hazard prevention.

Some building laws also permit a demolition order if a legally erected building that is no longer in use begins to deteriorate (without already being in danger of collapsing) and no interest can be claimed in maintaining the building (cf. Section 80 para. 2 Brandenburg Building Code (BbgBO 2018) and Section 79 para. 2 Bremen Building Code – BremLBO 2018). As part of the demolition, structural foundations that significantly restrict water permeability of the soil must be removed (Section 82 i.V.m. Section 10 Abs. 4 Rhineland-Palatinate Building Code – LBauO RP 1998). Ideally, such regulations should also be introduced to other federal states.

5.5.2 Greening and Water Infiltration Requirements: In Particular the Prohibition of Gravel Gardens

The requirement to unseal land can also result from greening regulations for unbuilt property areas anchored in the state building codes⁴⁸ or independent greening laws.⁴⁹ Usually these also stipulate that the water infiltration capacity of sites be untouched or restored. Compliance with these regulations is supposed to be monitored. In the event of non-compliance, the building supervisory authorities must work to ensure that the sealing is removed and the green space (re-)restored. These regulations, however, are not consistently enforced.

A special problem are so-called gravel gardens, which conflict with the goals of soil protection and climate adaptation.⁵⁰ Nevertheless, such garden designs are becoming increasingly popular and often tolerated by the authorities. The newly introduced Section 21a in the amended Baden-Württemberg Nature Conservation Act (NatSchG BW ÄndG 2020) stipulates that such gardens violate the greening requirement under building law. This makes it easier for the authorities to issue a removal order due to the illegality of the project.⁵¹ Whether this results in an obligation to remove existing gravel gardens is, however, questionable and requires further legal clarification.⁵²

5.6 Impact Mitigation Regulation

In practice, most unsealing measures are implemented on the basis of the so-called “impact mitigation regulation” (cf. Sections 13 ff. Federal Nature Conservation Act – BNatSchG 2009). This stipulates that interventions in nature and the landscape (and thus also the use of soils) must be compensated. This can be achieved through various measures such as tree planting, the creation of biotopes or soil unsealing. Unsealing measures are considered “classic” compensatory measures: these restore the functional capacity of the soil, enable the creation of new habitats for plants and animals or natural succession and, if necessary, can help eliminate existing damage to the landscape.⁵³

A special obligation to examine possible unsealing measures follows from Sections 15 para. 3 BNatSchG. According to this regulation, land particularly suitable for agricultural activities shall only be used for compensation measures where no viable alternatives exist. In order to fulfil this requirement, it must be examined first and foremost whether compensation can be achieved through unsealing measures, i.e. through the removal of existing sealing and the demolition of building structures that are no longer required. This is intended to prevent stocks of agricultural and forestry land being used for compensation measures.⁵⁴

Unsealing measures thus have top priority, at least in regards to preventing the use of agricultural and forestry land for compensation. However, these measures can be rather costly.⁵⁵ To provide encouragement, some federal states have developed ideas to promote unsealing measures such as offering discounts on the amount of required compensation.⁵⁶ These approaches have been taken up by the Federal Compensation Ordinance (BKompV 2020), which applies to interventions by federal projects: According to Section 8 para. 3 BKompV, unsealing measures are to be given priority for funding over other compensation measures (e.g. upgrading of existing habitats).

In addition to the appropriate methodological considerations of unsealing measures in the assessment of impact compensation, other factors will influence their realisation, e.g. information on the quality and availability of areas to be unsealed, which should be systematically recorded and made available within the framework of unsealing registers.⁵⁷ Another challenge is the procurement of land for compensation measures, for which the polluter has only limited options. Here, eco-accounts can offer a solution. By enabling the pooling of compensatory sites, several construction projects can collaborate in order to make an unsealing measure feasible (cf. Section 16 BNatSchG).⁵⁸

It should be noted, however, that unsealing measures within the framework of the impact mitigation regulation at best contribute to maintaining the status quo. This is because they presuppose new sealing elsewhere, which must be compensated for. In addition, the special regulations for urban land use planning (cf. Section 18 BNatSchG and Sections 1a, Section13a and 13b BauGB) restrict the applicability of the impact mitigation regulation in settled areas. We believe this should be changed.⁵⁹

5.7 Water Regulations

Water law offers a variety of starting points for unsealing measures. These have positive effects not only on the ecological quality of water bodies but also on groundwater recharge, natural flood protection and rainwater management.

5.7.1 Regulations on Water Quality

The EU Water Framework Directive (WFD 2000), which in Germany is implemented within the Federal Water Act (WHG 2009), can promote unsealing by requiring renaturation measures to achieve good water status (cf. Sections 27 ff. WHG). This is because the required good status also applies to ecological factors and is oriented towards a natural status. Examples of this are the renaturation of obstructed river banks or the opening up of piped water bodies. Unsealing measures are part of the programmes of measures specified by Section 82 WHG and which are implemented by the water authorities.⁶⁰

It should be noted, however, that these measures are primarily limited to the watercourse corridor, i.e. they do not impact a broader area.⁶¹ Furthermore, a major challenge for the authorities is to procure the necessary areas for unsealing measures. Implementation also depends on the availability of financial resources. In addition, such conversion measures are considerably delayed by the planning approval procedures that are sometimes required. Generally, municipalities could benefit from additional funding for land acquisition.⁶²

5.7.2 Regulations on Flood Risk Management

Unsealing measures can also be triggered by plans to create near-natural flood protection. Soil unsealing promotes water retention, which in turn helps prevent flooding. Such measures can be defined in the flood risk management plans under the EU Floods Directive (FD 2007) (cf. Section 75 WHG).⁶³ Unsealing measures are also promoted by the provision of Section 77 para. 2 WHG, according to which former floodplains should be restored as far as possible. Further, such measures can provide the necessary compensation for newly sealed areas in floodplains (Section 77 para. 1, Sections 78 ff. WHG).

In the case of designated “flood generation areas” (Hochwasserentstehungsgebiete) under Section 78d WHG, compensation for the loss of retention space due to sealing and changes of use above a certain size is also required in the hinterland (para. 4 to 6). In addition, there is a general unsealing requirement in these areas (para. 3), which, however, is not directly enforceable.⁶⁴ The flood generation areas are located in mountainous areas at the headwaters of watercourses, i.e. where the floods originate. Until now, such areas have only been designated in one federal state, namely Saxony. Their implementation should also be extended to other states.

5.7.3 Regulation of Wastewater Management

Rainwater disposal and management is a crucial field of action for climate adaptation, one which has hitherto been insufficiently standardised. Here the aim should be to retain water in areas where unsealed soil can reduce the water load. The principle of Section 55 para. 2 WHG points in this direction, according to which rainwater should be infiltrated close to the site where it falls, trickled away or discharged directly or via a sewer system into a body of water without being mixed with wastewater.

However, Section 55 para. 2 WHG is not directly enforceable; instead, it requires further specification, which is not provided in the WHG. In particular, a clear responsibility for stormwater management should be named in the law and the responsible institution (e.g. urban drainage) given funding to fulfil this task.⁶⁵ Following the example of some water regulations in Germany’s federal states, provisions should be included that standardise the authorisation of property owners to manage their own rainwater.⁶⁶ Conceptually, a drainage plan is required that is oriented towards decentralised rainwater management and coordinated with the urban development uses.⁶⁷

The obligation to charge property owners for rainwater disposal provides a financial incentive for them to disconnect unsealed areas (for which rainwater infiltration is guaranteed) from the sewage system. This allows the owner to save on wastewater fees. A calculation of fees according to the “split wastewater fee” also provides a financial incentive for unsealing.⁶⁸ Accordingly, wastewater charges are not based on water consumption but rather on the size of the built-up or artificially paved area of the property from which rainwater runs off. Unsealing can thus help reduce charges for wastewater disposal.

5.8 Climate Laws

Unsealing measures are also necessary for reasons of climate protection and adaptation: intact soils are carbon reservoirs and thus help counteract global warming. These facts should be stated more clearly in the description of soil functions in Section 2 para. 2 BBodSchG.⁶⁹ The evaporation, infiltration and cooling effects of the soil also contribute to climate adaptation.⁷⁰ The legal framework for climate-impacting measures is provided by the Federal Climate Protection Act (KSG 2019) and the climate (protection) laws of the federal states (as far as available).⁷¹ Climate targets are implemented through climate protection plans and adaptation strategies. These should also address the issue of unsealing and list appropriate measures.

It is particularly important to improve soil functions in those urban areas susceptible to overheating.⁷² In theory, the climate (protection) laws of the states (Länder) can promote appropriate measures, especially by including them in municipal climate (protection) concepts. It would be better if there were an obligation rather than just a recommendation to draw up such concepts.⁷³ A further improvement would be regulate the interlinking of municipal climate concepts with urban land use planning, thereby boosting the likely implementation of measures.⁷⁴ In addition, the law should make clear that climate adaptation and protection are municipal tasks, and that local authorities are obliged to ensure climate-adapted urban development.

6 Key Messages and Recommendations

Our investigation has revealed a discrepancy between the various benefits of unsealing, on the one hand, and the scarce availability of unsealing potentials and various obstacles for the realisation of unsealing measures, on the other. This suggests a need for inventories of prerequisites and opportunities as well as their appropriate combination: For this, climate adaptation requirements in residential areas have to be investigated at district level to identify the specific prerequisites. Simultaneously, unsealing potentials have to be identified and characterised to determine the specific opportunities to contribute to climate adaptation goals. Finally, consistent and balanced concepts for inner development at municipal or district level have to be created and implemented to address the identified climate adaptation requirements by the use of the identified opportunities for unsealing, while closely interlinking these with further measures. As unsealing measures can be expensive, it could be useful to rank these in terms of their benefit for climate adaptation along with their costs.

Unsealing must not be seen as an end in itself but rather as one element in an overall development approach that encompasses the creation of green spaces and encouragement of social urban development as well as decontamination of sites. These goals may be facilitated by incentives. To boost the implementation of unsealing measures, a sufficient supply of good information and advice is important, not least in order to include a large number of different actors, such as lessors and tenants of housing or commercial areas. As key players in unsealing, Germany’s municipalities need to be empowered in terms of financial and personnel resources to be able initiate unsealing measures amongst local residents as well as to set a good example themselves.

The various legal instruments currently available to promote unsealing are rather inefficient and impractical. In particular, the unsealing obligations specified in Section 179 BauGB and Section 5 BauGB should be clearly distinguished, simplified and expanded in order to reduce the existing enforcement deficit. Their application should be supported by dedicated funds. The contribution of unsealed soils to climate protection and climate adaptation should be more strongly expressed in soil protection law. In addition, the climate protection laws of the federal states should require mandatory climate adaptation planning by the municipalities.

The impact mitigation regulation already plays an important role in the implementation of unsealing measures. Its legal provisions should be strengthened to ensure that newly sealed areas are compensated by the unsealing of other sites. Last but not least, water law can promote the implementation of unsealing measures in various ways. In view of the challenges of climate change, it is essential that we create an integrated system of urban water management, one which does not just consider wastewater disposal but also coordinates all water-related measures (i.e. water quality, flood protection, groundwater recharge). Here legal prerequisites should be created, aimed at optimising the system for the greatest possible ecological effect.