**3. Sustainable buildings as contributors to the circular economy in urban regeneration**

The benefits of energy renovation in urban regeneration are not limited to energy savings alone. It is broadly recognised to also benefit people and the environment. Good indoor climate, thermal comfort, acoustic performance and daylight, for example, improve the health and wellbeing of inhabitants and positively affect the productivity in schools, hospitals and of workers in general [5]. Taking a life-cycle approach when considering the materials used in buildings can reduce the environmental impact of the buildings in a city. Moreover, valuing the potential for today and the future creates better economic value. Not surprisingly, there is clear trend towards a more sustainable design and renovation of buildings.

Urban centres are particularly impacted by waste from construction and demolition activities (CDW). The Ellen MacArthur Foundation has identified the built environment to be one of the key sectors of structural waste [2]. For example, in Europe, the average office is used only 35–50% of the time, even during working hours. The UN-IRP study called 'The Weight of Cities' [1] shows that the global 'domestic material consumption' (DMC) of raw materials (including sand, gravel, iron ore, coal and wood) is likely to be in the range of 8–17 tonnes per capita per year at 2050, assuming material use per capita will stabilise in developing countries at lower levels than today's developed countries. However, a DMC range of 6–8 tonnes per capita per year has been proposed as an indicative target for sustainable resource consumption [7]. In 2050, urban mining should be the main source of building materials in cities, with implications for storage, logistics and costs, while citizens will increasingly require a nontoxic environment.

Cities are well placed to tackle the resource and waste issues in the construction sector, with their high concentration of resources, capital, data, and talent over a small geographic territory. Closing the loop in the construction sector and improving design would lead to major benefits in sustainability and in quality of life. This could further support urban policymakers in achieving their objectives when it comes to carbon emissions, mobility, indoor air quality [2] and working towards a non-toxic environment. This transition towards a sustainable, lowcarbon and resource-efficient economy—called a 'circular economy'—is considered vital to future-proofing cities and improving quality of life for citizens [8]. Our world economy is considered to be only 9.1% circular at the moment [37]. The Circle Economy states that 'Closing the circularity gap serves the higher objective of preventing further and accelerated environmental degradation and social inequality'—especially since housing and infrastructure needs represent the largest resource footprint [37]. They believe that a circular economy approach enables cities to take practical steps to help reduce emissions, create new jobs and strengthen industries and competitiveness, as well as enhance the health and wellbeing of its citizens. Sustainable buildings contribute to this transition.

the residents, and minimise virgin material use. It will be built using efficient construction techniques, and will be highly utilised thanks to shared, flexible and modular office spaces and housing. Components of buildings will be maintained and renewed when needed, while buildings will be used where possible to generate, rather than consume, power and food by facilitating closed loops of water, nutrients, materials, and energy, to mimic natural cycles' [2]. This description shows several aspects of buildings in circular cities, centred around the

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Today, buildings are often created with only one function in mind, like being a school, an office or a multi-family home. When societal needs or user preferences change, these monofunctional buildings usually become outdated or even obsolete—resulting in a high rate of building vacancy and premature demolition. Those buildings, no longer suitable for use, are considered as waste and are thus treated as such. By applying circularity principles both to the existing building stock and to new buildings, the concept of waste can be eliminated.

This principle is elaborated in the European project Buildings as Material Banks (BAMB) [14]. According to the BAMB vision, the term 'Buildings as Material Banks' refers to a materialised investment: "the building itself is considered as a materialised savings account for material resources, through which building materials, products and components are temporarily 'deposited' into a functional element or part of the building. When socio-economic conditions are favourable, (a part of) the materials, products and components may be retrieved for another investment, that is: another building or another high-quality application. Seeing material resources as a temporary way of materialising investments opens the door to a wide range of circular business models, in which economic and environmental value is conserved and created through the reuse of materials, products, components and buildings, while (performance-based) services are provided to support the daily life of (end)

Three major systemic changes have been identified to support the BAMB vision:

Both the existing building stock and the new buildings are interlinked in urban regeneration.

The growing demand for and the scarcity of resources, less space for landfill and the challenges regarding the movement of resources through the city all create a push towards an approach of using the materials and products in existing buildings in the most optimal way. 'Urban mining' is mentioned in several sources as a necessary approach in circular cities [1, 15]. At the same time, it is identified that there are insufficient economic and regulatory incentives as well as a lack of trust in the quality and availability of secondary materials to create a market for recycled building materials [15, 16]. A lack of demand for reused or recycled products in

basic assumption that buildings are '*material banks*'.

users."

• Change in design culture • Change in value definition

• Change in collaboration across all actors

**3.3. Resources from the existing building stock**

#### **3.1. Sustainable buildings**

There is not one definition for 'sustainable buildings'. However, what all descriptions have in common is that a holistic approach is taken in the design, construction and demolition processes to minimise the buildings' impact on the environment, the occupants and the community and to maintain economic value. When it comes to the environmental aspect of sustainable buildings, the World Green Building Council describes a 'green' building as 'a building that, in its design, construction or operation, reduces or eliminates negative impacts, and can create positive impacts, on our climate and natural environment. Green buildings preserve precious natural resources and improve our quality of life' [9].

There are several features that can make a building 'green' and additional features to make them more sustainable. Environmental features pertain to resources like materials, water and land and energy and carbon, as well as pollution of air, water and soil. Health-related features pertain to indoor climate, acoustic performance and thermal comfort. Additional features could be related to risks like earthquakes and fires—addressing the resources that the building represents.

Several tools and sustainable building rating schemes are in place to assess the sustainability of buildings. Examples of European-wide used commercial rating schemes are BREEAM, DGNB, HQE and LEED, but local schemes exist too. The European Commission recently launched a beta version of a framework for sustainable buildings called Level(s) [10], which describes the basic relevant features for the European housing stock. In the Netherlands [11] and France [12], building legislation is in place to assess environmental building features, and in Germany [13], a similar approach is taken for the Green Public Procurement of buildings.

The key step to a sustainable design is to reduce the amount of energy needed to operate a building. Typically, 80% of the energy used in a building results from heating, cooling, hot water and lighting, while the remaining 20% is used for construction materials, transport and demolition. However, moving towards a more energy-efficient building stock will change this ratio over time—putting more focus on the materials and the 'beyond energy' features of sustainability. Environmental assessments of buildings are therefore based on a life-cycle approach using various environmental indicators.

#### **3.2. Buildings as Material Banks**

Considering that a so-called circular economy is vital to future-proofing cities and improving the quality of life of citizens, according to [8], organisations need to stimulate the development of a circular economy by developing concepts for cities to become more circular. The Ellen MacArthur Foundation describes the built environment in 'circular cities' as 'designed in a modular and flexible manner, sourcing healthy materials that improve the life quality of the residents, and minimise virgin material use. It will be built using efficient construction techniques, and will be highly utilised thanks to shared, flexible and modular office spaces and housing. Components of buildings will be maintained and renewed when needed, while buildings will be used where possible to generate, rather than consume, power and food by facilitating closed loops of water, nutrients, materials, and energy, to mimic natural cycles' [2]. This description shows several aspects of buildings in circular cities, centred around the basic assumption that buildings are '*material banks*'.

Today, buildings are often created with only one function in mind, like being a school, an office or a multi-family home. When societal needs or user preferences change, these monofunctional buildings usually become outdated or even obsolete—resulting in a high rate of building vacancy and premature demolition. Those buildings, no longer suitable for use, are considered as waste and are thus treated as such. By applying circularity principles both to the existing building stock and to new buildings, the concept of waste can be eliminated.

This principle is elaborated in the European project Buildings as Material Banks (BAMB) [14]. According to the BAMB vision, the term 'Buildings as Material Banks' refers to a materialised investment: "the building itself is considered as a materialised savings account for material resources, through which building materials, products and components are temporarily 'deposited' into a functional element or part of the building. When socio-economic conditions are favourable, (a part of) the materials, products and components may be retrieved for another investment, that is: another building or another high-quality application. Seeing material resources as a temporary way of materialising investments opens the door to a wide range of circular business models, in which economic and environmental value is conserved and created through the reuse of materials, products, components and buildings, while (performance-based) services are provided to support the daily life of (end) users."

Three major systemic changes have been identified to support the BAMB vision:

• Change in design culture

represent the largest resource footprint [37]. They believe that a circular economy approach enables cities to take practical steps to help reduce emissions, create new jobs and strengthen industries and competitiveness, as well as enhance the health and wellbeing of its citizens.

There is not one definition for 'sustainable buildings'. However, what all descriptions have in common is that a holistic approach is taken in the design, construction and demolition processes to minimise the buildings' impact on the environment, the occupants and the community and to maintain economic value. When it comes to the environmental aspect of sustainable buildings, the World Green Building Council describes a 'green' building as 'a building that, in its design, construction or operation, reduces or eliminates negative impacts, and can create positive impacts, on our climate and natural environment. Green buildings

There are several features that can make a building 'green' and additional features to make them more sustainable. Environmental features pertain to resources like materials, water and land and energy and carbon, as well as pollution of air, water and soil. Health-related features pertain to indoor climate, acoustic performance and thermal comfort. Additional features could be related to risks like earthquakes and fires—addressing the resources that the build-

Several tools and sustainable building rating schemes are in place to assess the sustainability of buildings. Examples of European-wide used commercial rating schemes are BREEAM, DGNB, HQE and LEED, but local schemes exist too. The European Commission recently launched a beta version of a framework for sustainable buildings called Level(s) [10], which describes the basic relevant features for the European housing stock. In the Netherlands [11] and France [12], building legislation is in place to assess environmental building features, and in Germany [13], a similar approach is taken for the Green Public Procurement of buildings. The key step to a sustainable design is to reduce the amount of energy needed to operate a building. Typically, 80% of the energy used in a building results from heating, cooling, hot water and lighting, while the remaining 20% is used for construction materials, transport and demolition. However, moving towards a more energy-efficient building stock will change this ratio over time—putting more focus on the materials and the 'beyond energy' features of sustainability. Environmental assessments of buildings are therefore based on a life-cycle

Considering that a so-called circular economy is vital to future-proofing cities and improving the quality of life of citizens, according to [8], organisations need to stimulate the development of a circular economy by developing concepts for cities to become more circular. The Ellen MacArthur Foundation describes the built environment in 'circular cities' as 'designed in a modular and flexible manner, sourcing healthy materials that improve the life quality of

preserve precious natural resources and improve our quality of life' [9].

Sustainable buildings contribute to this transition.

48 Sustainable Cities - Authenticity, Ambition and Dream

approach using various environmental indicators.

**3.2. Buildings as Material Banks**

**3.1. Sustainable buildings**

ing represents.


Both the existing building stock and the new buildings are interlinked in urban regeneration.

#### **3.3. Resources from the existing building stock**

The growing demand for and the scarcity of resources, less space for landfill and the challenges regarding the movement of resources through the city all create a push towards an approach of using the materials and products in existing buildings in the most optimal way. 'Urban mining' is mentioned in several sources as a necessary approach in circular cities [1, 15]. At the same time, it is identified that there are insufficient economic and regulatory incentives as well as a lack of trust in the quality and availability of secondary materials to create a market for recycled building materials [15, 16]. A lack of demand for reused or recycled products in combination with easy and cheap waste landfill and incineration hamper a sound business model throughout the construction chain.

The EU Construction and Demolition Waste Protocol [17] lists seven features for successful implementation of recycling:


Below, possible measures are discussed to keep material resources from buildings in the loop, for both the end of the building's life and the production of new products.

to be considered a 'non-waste' while guaranteeing current levels of protection of public health and the environment. It could be similar to the conditions for by-products of manufacturing processes [15], so that materials, such as bricks, fittings, doors, window panes, beams, etc., can be transported, stored, processed, and sold as products provided these

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Closing the route towards landfill and incineration must go together with boosting demand for reused products and secondary materials. The market for secondary raw materials is still immature, and several policy choices exist to boost this market. The policy toolkit report from the Ellen MacArthur Foundation [38] explains it as follows: 'There are two broad, complementary policymaking strategies that can help accelerate the circular economy. The first is to focus on fixing market and regulatory failures. The second is to actively stimulate market activity by, for example, setting targets, changing public procurement policy, creating collaboration platforms and providing financial or technical support to businesses. These approaches are complementary and policymakers can determine where to put the emphasis, taking inspira-

Within public procurement, cities have the power to set high thresholds for recycled content in their public tenders and encourage the use of reusable and recyclable materials and products. Other suggestions are fast-tracked approval of construction permits, easier access to capital and stimulating new ownership models of buildings and constructions [15]. Creating platforms for collaboration and promoting value networks instead of value chains could also be a role for cities, as shown by the city of Brussels in the BAMB project [14] and the Dutch city of Almere in their UpCycle City contest [19]. A key takeaway from the Amsterdam Circular is to work closely together with the private sector and research institutes to find new business

Note that there is a growing belief that markets should be created without government interference like subsidies and levies on raw materials but much more by facilitating the emer-

conditions are met.

*3.3.2. Measures to stimulate the use of secondary materials*

**Figure 3.** An illustration of the application of current definitions of waste [18].

tion from the most applicable aspects of both approaches'.

gence of business models that can work over time.

models in the context of existing strategies, such as green procurement.

#### *3.3.1. Measures at the end of the life of a building*

Upon demolition, budget constraints do not allow for (often more labour-intensive) the deconstruction of separate material fractions. The limited space for collection of many fractions is a further barrier in cities—requiring a more sophisticated after-sorting. Toxic legacy materials require specific attention if products must be reused or recycled. In many countries, landfill or incineration are still easily accessible and are too cheap to incentivise economically feasible alternatives. Separation and recycling only work business-wise in countries or regions with strict landfill bans for recyclable materials—like in Denmark or the Netherlands. This must be combined with (legal) requirements or incentives for buildings to be stripped before their frames can be demolished following an assessment (pre-demolition audit). This can create an entire service sector of SMEs with expertise in the removal of tiles, flooring, window frames, insulation material, sanitary items, lighting, etc. The Rotor in Flanders, Belgium, is an example of such a specialised company, as presented on the event from GLOBE-EU [15].

Cities can lead the way by discouraging landfill and incineration of construction and demolition waste and stimulating deconstruction, sorting and facilitate reuse platforms like Rotor. Local urban initiatives also contribute to social cohesion and job creation [8].

To facilitate this further, it is suggested to open up the European concept of waste [15, 18], in which 'preparing for reuse and recycling' is part of the waste definition, as shown in the left-hand side of **Figure 3**. On the right-hand side, an additional category is added between non-waste and waste (refuse), representing a situation in which secondary resources for reuse and recycling will not be regarded as waste to better fit the circular economy. Cities will have much more room for policies and experiments, if the European Commission would issue guidelines on the conditions to be fulfilled for recovered or reclaimed materials

#### Buildings in Urban Regeneration http://dx.doi.org/10.5772/intechopen.81803 51

**Figure 3.** An illustration of the application of current definitions of waste [18].

combination with easy and cheap waste landfill and incineration hamper a sound business

The EU Construction and Demolition Waste Protocol [17] lists seven features for successful

Below, possible measures are discussed to keep material resources from buildings in the loop,

Upon demolition, budget constraints do not allow for (often more labour-intensive) the deconstruction of separate material fractions. The limited space for collection of many fractions is a further barrier in cities—requiring a more sophisticated after-sorting. Toxic legacy materials require specific attention if products must be reused or recycled. In many countries, landfill or incineration are still easily accessible and are too cheap to incentivise economically feasible alternatives. Separation and recycling only work business-wise in countries or regions with strict landfill bans for recyclable materials—like in Denmark or the Netherlands. This must be combined with (legal) requirements or incentives for buildings to be stripped before their frames can be demolished following an assessment (pre-demolition audit). This can create an entire service sector of SMEs with expertise in the removal of tiles, flooring, window frames, insulation material, sanitary items, lighting, etc. The Rotor in Flanders, Belgium, is an example of such a specialised company, as presented on the event from GLOBE-EU [15]. Cities can lead the way by discouraging landfill and incineration of construction and demolition waste and stimulating deconstruction, sorting and facilitate reuse platforms like Rotor.

**2.** Implement mandatory pre-demolition waste audits (identify quality and quantity). **3.** Enforce traceability of waste to establish confidence (especially during transport).

**5.** Manage quality of recycled waste (same quality standards as virgin materials).

**7.** Create open markets through leading example of public procurement.

for both the end of the building's life and the production of new products.

Local urban initiatives also contribute to social cohesion and job creation [8].

To facilitate this further, it is suggested to open up the European concept of waste [15, 18], in which 'preparing for reuse and recycling' is part of the waste definition, as shown in the left-hand side of **Figure 3**. On the right-hand side, an additional category is added between non-waste and waste (refuse), representing a situation in which secondary resources for reuse and recycling will not be regarded as waste to better fit the circular economy. Cities will have much more room for policies and experiments, if the European Commission would issue guidelines on the conditions to be fulfilled for recovered or reclaimed materials

model throughout the construction chain.

50 Sustainable Cities - Authenticity, Ambition and Dream

**1.** Ban landfilling of construction and demolition waste.

**4.** Urban planning (recycling facilities within city limits).

**6.** Environmental management along the entire value chain.

*3.3.1. Measures at the end of the life of a building*

implementation of recycling:

to be considered a 'non-waste' while guaranteeing current levels of protection of public health and the environment. It could be similar to the conditions for by-products of manufacturing processes [15], so that materials, such as bricks, fittings, doors, window panes, beams, etc., can be transported, stored, processed, and sold as products provided these conditions are met.

#### *3.3.2. Measures to stimulate the use of secondary materials*

Closing the route towards landfill and incineration must go together with boosting demand for reused products and secondary materials. The market for secondary raw materials is still immature, and several policy choices exist to boost this market. The policy toolkit report from the Ellen MacArthur Foundation [38] explains it as follows: 'There are two broad, complementary policymaking strategies that can help accelerate the circular economy. The first is to focus on fixing market and regulatory failures. The second is to actively stimulate market activity by, for example, setting targets, changing public procurement policy, creating collaboration platforms and providing financial or technical support to businesses. These approaches are complementary and policymakers can determine where to put the emphasis, taking inspiration from the most applicable aspects of both approaches'.

Within public procurement, cities have the power to set high thresholds for recycled content in their public tenders and encourage the use of reusable and recyclable materials and products. Other suggestions are fast-tracked approval of construction permits, easier access to capital and stimulating new ownership models of buildings and constructions [15]. Creating platforms for collaboration and promoting value networks instead of value chains could also be a role for cities, as shown by the city of Brussels in the BAMB project [14] and the Dutch city of Almere in their UpCycle City contest [19]. A key takeaway from the Amsterdam Circular is to work closely together with the private sector and research institutes to find new business models in the context of existing strategies, such as green procurement.

Note that there is a growing belief that markets should be created without government interference like subsidies and levies on raw materials but much more by facilitating the emergence of business models that can work over time.

#### **3.4. New buildings prepared for the future**

New buildings can be requested to be designed and built in a 'circular' way. Better information about a building would help close the loop, and lower costs as worst-case scenarios in the future need no longer apply. Digital tools like BIM and cloud-based platforms of comprehensive data on buildings can already be used for today's buildings.

are not well defined yet and are suggested for different purposes, including renovation and circular economy, but they could also be a helpful tool for risk evaluation in case of, e.g. a fire or natural disaster. An example of a building passport in the context of the circular economy is Madaster [24]. It is Madaster's mission to eliminate waste by providing materials with an identity. The Madaster Platform facilitates the registration, organisation, storage and exchange of data of buildings over their lifespan, thereby becoming a public, online library of materials. The concept of building passports needs further development to be used in a consistent way on a large scale, but the principle clearly has a place when developing more

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It could be helpful in procurement and design processes if a definition of 'circular buildings' would exist. This is not the case, as the possibility of having a singular definition is being challenged, and the idea that 'circular buildings' are multidimensional is being considered.

Several suggestions have been made, for example in [2, 14, 20, 25]. Common themes in the descriptions of 'circular buildings' are energy efficiency, the materials applied in the building (their use of resources, reuse and recycling options and the waste they cause) and a design

Such a 'circular building' is not similar to a 'sustainable building': sustainability is to be assessed, but a circular building puts more focus on design and saving resources while not compromising sustainability. For the environmental assessment, an LCA approach as described before is indispensable. LCA reveals any trade-offs of specific measures targeting specific issues, like resources or waste in the circular economy. In this regard, it is important to remember that the circular economy and circular buildings target a goal—usually of reducing resource consumption and waste—*but 'circularity' is not a goal in itself*! Not all circular concepts will result in environmental benefits in the end. It can be questioned, for example, if materials should be recycled into products that are not recyclable anymore—a pitfall for some innovative products

The term resilience has been around for centuries and has been used in engineering, ecology

Several definitions of the concept of resilience can be found in the literature, for example, in publications from Timmerman [26] and Holling [27]. The UN Office for Disaster Risk

*"The ability of a system, community or society exposed to hazards to resist, absorb, accommodate, adapt to, transform and recover from the effects of a hazard in a timely and efficient manner, including through the preservation and restoration of its essential basic structures and functions through risk* 

and psychology before coming to civil protection and disaster risk reduction.

Reduction came with the following definition in 2009:

that enables long-term benefits from the building and its components.

circular buildings in an urban context.

*3.4.3. What is a circular building?*

made from waste.

**4.1. Resilience**

**4. Resilient buildings**

*management." [28].*

#### *3.4.1. Reversible design and open building concept*

Modularity and flexibility are mentioned by the Ellen MacArthur Foundation as key to circular buildings in cities [2]. A Dutch Architect Manifesto refers to both adaptability/flexibility and easy-to-dismantle construction [20]. Research on such reversible building designs is broadly available, as shown in the BAMB project [14] and the EU-based project [21], although it is not a common practice yet. The Flanders region in Belgium has applied these principles in their policy for 'Building for change', a policy that anticipates on future changes in building use, as presented in [15].

An 'open building' concept is based on similar principles, thereby taking the perspective of ownership. This seems especially interesting in the urban environment, with many rental buildings and apartment blocks. This concept suggests that buildings need to be looked at from the perspective of the owner (long-term client) and the tenant (short-term client), respectively. The diverging interests of these two owners make it important to treat them separately (two-step housing). Investors in real estate are interested in the 'shell' of a building, not the interior outfit, which is considered more of a liability. Cities in Japan are more advanced in the implementation of an open building concept which recognises that people need to adapt their living space according to changing circumstances. An entire industry sector has emerged around the remodelling of homes based on circularity principles: easy-to-remodel interiors, reuse of components and floor space tailored to family size [15, 22].

Encouraging innovation by adding reversible design to the conditions for competitive bidding on public works contracts and the development of EU-wide indicators and standards for reversible building design would go a long way towards more sustainable building design and use.

#### *3.4.2. Information throughout the chain*

Information on the materials in a building and on the building itself over time facilitates a building to be a material bank in future. The digitalisation of society will support this.

Material passports are based on the principle that one should know what type of materials is applied in the building. Information on content should reveal potential future legacies of hazardous materials that could hamper recycling. Further information, e.g. on recyclability, is suggested [14, 23]. It is likely that building information modelling (BIM) will also play a role in the transfer of environmental product information through the chain, as it already fulfils this role for technical characteristics.

Whereas material passports are typically connected to products and their manufacturers, building passports could play a role in city policies. Building passports provide information on the whole building design, its maintenance, refurbishments, etc. Building passports are not well defined yet and are suggested for different purposes, including renovation and circular economy, but they could also be a helpful tool for risk evaluation in case of, e.g. a fire or natural disaster. An example of a building passport in the context of the circular economy is Madaster [24]. It is Madaster's mission to eliminate waste by providing materials with an identity. The Madaster Platform facilitates the registration, organisation, storage and exchange of data of buildings over their lifespan, thereby becoming a public, online library of materials. The concept of building passports needs further development to be used in a consistent way on a large scale, but the principle clearly has a place when developing more circular buildings in an urban context.

#### *3.4.3. What is a circular building?*

**3.4. New buildings prepared for the future**

52 Sustainable Cities - Authenticity, Ambition and Dream

*3.4.1. Reversible design and open building concept*

use, as presented in [15].

*3.4.2. Information throughout the chain*

this role for technical characteristics.

sive data on buildings can already be used for today's buildings.

reuse of components and floor space tailored to family size [15, 22].

New buildings can be requested to be designed and built in a 'circular' way. Better information about a building would help close the loop, and lower costs as worst-case scenarios in the future need no longer apply. Digital tools like BIM and cloud-based platforms of comprehen-

Modularity and flexibility are mentioned by the Ellen MacArthur Foundation as key to circular buildings in cities [2]. A Dutch Architect Manifesto refers to both adaptability/flexibility and easy-to-dismantle construction [20]. Research on such reversible building designs is broadly available, as shown in the BAMB project [14] and the EU-based project [21], although it is not a common practice yet. The Flanders region in Belgium has applied these principles in their policy for 'Building for change', a policy that anticipates on future changes in building

An 'open building' concept is based on similar principles, thereby taking the perspective of ownership. This seems especially interesting in the urban environment, with many rental buildings and apartment blocks. This concept suggests that buildings need to be looked at from the perspective of the owner (long-term client) and the tenant (short-term client), respectively. The diverging interests of these two owners make it important to treat them separately (two-step housing). Investors in real estate are interested in the 'shell' of a building, not the interior outfit, which is considered more of a liability. Cities in Japan are more advanced in the implementation of an open building concept which recognises that people need to adapt their living space according to changing circumstances. An entire industry sector has emerged around the remodelling of homes based on circularity principles: easy-to-remodel interiors,

Encouraging innovation by adding reversible design to the conditions for competitive bidding on public works contracts and the development of EU-wide indicators and standards for reversible building design would go a long way towards more sustainable building design and use.

Information on the materials in a building and on the building itself over time facilitates a

Material passports are based on the principle that one should know what type of materials is applied in the building. Information on content should reveal potential future legacies of hazardous materials that could hamper recycling. Further information, e.g. on recyclability, is suggested [14, 23]. It is likely that building information modelling (BIM) will also play a role in the transfer of environmental product information through the chain, as it already fulfils

Whereas material passports are typically connected to products and their manufacturers, building passports could play a role in city policies. Building passports provide information on the whole building design, its maintenance, refurbishments, etc. Building passports

building to be a material bank in future. The digitalisation of society will support this.

It could be helpful in procurement and design processes if a definition of 'circular buildings' would exist. This is not the case, as the possibility of having a singular definition is being challenged, and the idea that 'circular buildings' are multidimensional is being considered.

Several suggestions have been made, for example in [2, 14, 20, 25]. Common themes in the descriptions of 'circular buildings' are energy efficiency, the materials applied in the building (their use of resources, reuse and recycling options and the waste they cause) and a design that enables long-term benefits from the building and its components.

Such a 'circular building' is not similar to a 'sustainable building': sustainability is to be assessed, but a circular building puts more focus on design and saving resources while not compromising sustainability. For the environmental assessment, an LCA approach as described before is indispensable. LCA reveals any trade-offs of specific measures targeting specific issues, like resources or waste in the circular economy. In this regard, it is important to remember that the circular economy and circular buildings target a goal—usually of reducing resource consumption and waste—*but 'circularity' is not a goal in itself*! Not all circular concepts will result in environmental benefits in the end. It can be questioned, for example, if materials should be recycled into products that are not recyclable anymore—a pitfall for some innovative products made from waste.
