Johannes Staudt, Carsten Schade, Catherine Steiner, Werner Lang

DOI: https://doi.org/10.18154/RWTH-2025-05383

Keywords: circularity; building transformation; building use; neighborhoods; early planning phases

The pressing issues of our time such as the climate crisis, resource scarcity, and affordable housing demonstrate the challenges of balancing human needs with planetary boundaries (Raworth 2017). The built environment is a major contributor to this polycrisis (International Energy Agency 2019). In architecture and urban planning circular approaches and adaptive reuse have been proposed to address these challenges. The adaptation and activation of existing buildings can contribute to sustainability by increasing efficiency and supporting uses that advance social and environmental transformation. Although refurbishment and conversion have been important topics for years, demolition and new construction still dominate in practice. The New European Bauhaus (NEB) initiative proposes a framing that outlines clear ecological and social sustainability objectives to develop new design and planning approaches that link residents‘ everyday lives with the European goal of climate neutrality (European Commission 2023). Our research is part of the NEB project Creating NEBourhoods Together. The challenges of transforming large, monofunctionally used office buildings were the impetus for our subproject Circular Neuperlach. This practice-oriented research explores transformation projects as catalysts for regenerative urban development. We develop a prototypical planning approach together with actors from city administration, the real estate industry, planning, design, and civil society. The methodology is intended to address the challenges and aspirations of multiple stakeholders and make the shared benefits of the transformation of existing buildings and neighborhoods negotiable.

Drawn from this work, the present article proposes the concept of circular uses as a way to think about the transformation of existing buildings and spaces into catalysts of positive change in the urban environment. In doing so, it builds on the discourse of urban circularity, which looks at cities as complex systems of resource and energy flow that span multiple scales, from materials and components to buildings, neighborhoods, cities, and regions. Whereas much of this discourse focuses on material recycling and reuse, our contribution is to highlight the use of buildings as a powerful but underexplored means of fostering circularity in these urban systems. We identify four types of circular use: continued use, change of use, intensified use, and multiple use. Presenting our findings from a series of interviews with experienced actors, we discuss strategies for and challenges to the implementation of circular uses, as well as the role of urban context, planning and design process, and regulatory innovations.

This overview situates our research within ongoing debates and evolving frameworks related to circular economy, circularity, and adaptive reuse. It explores how these concepts are used in both academic literature and professional practice. Particular attention is given to conceptual models of circularity to define a suitable concept for building transformation as a critical lever in sustainable urban development.

While often used interchangeably, circular economy and circularity differ in scope and underlying intent. In architectural and urban planning discourse, this distinction matters: circular economy has largely been framed within existing paradigms of growth and tends to prioritize material flows, recycling, efficiency, and component reuse in new construction (Ghisellini et al. 2016; Kirchherr et al. 2017; Korhonen et al. 2018; Pomponi and Moncaster 2017). In contrast, circularity focuses on how human systems, including buildings, infrastructures, and cities, can operate within planetary boundaries (Leising et al. 2018; Schröder et al. 2020; Cheshire 2024). As a concept of regeneration, circularity also fore-grounds intergenerational equity, affirming values of social justice, long-term adaptability, and ecological regeneration, which are not well addressed in dominant circular economy narratives (Williams 2019). It calls for broader transformation and is related to frameworks such as postgrowth economics (Jackson 2017; Petschow et al. 2020), regenerative urbanism (Girardet 2015), and doughnut economics (Raworth 2017). Regenerative design echoes these approaches, moving from minimizing harm to actively restoring ecosystems and enhancing the socioecological fabric of urban life (Mang and Reed 2012).

Hierarchical models of circularity (e.g., Potting et al. 2017; Figure 1) distinguish between lower-level strategies like recycling and higher-level ones such as sufficiency, regeneration, and systemic change. Recent expansions of these models emphasize the regeneration of socioecological systems (Loza Adaui 2024). The concept of circularity also includes considerations of time and space, addressing both the speed and scale at which resources flow through a system (Bocken et al. 2016).

The model of shearing layers by Stewart Brand (1995; Figure 2) and Frank Duffy describes buildings as systems of layers with different rates of change. Designing with these varying lifespans in mind enables adaptability and long-term sustainability by allowing faster-changing layers to be modified without disrupting slower ones. The model focuses on physical obsolescence. Additional criteria for obsolescence have been identified: economic, functional, technological, social, legal, and political (Conejos et al. 2013). Emotional durability (Bocken et al. 2016) addresses acceptance of existing buildings, as people care for buildings they identify with and join forces to hinder demolition.

The lifecycle of buildings spans decades or even centuries. To understand their long-term impact on urban environments a conceptualization that goes beyond individual buildings and captures their systemic effects is needed (Anders 2016). Buildings are not mere stocks of material or financial instruments of value creation; they also shape the social, economic, and cultural activities that allow cities to thrive (Eberhardt et al. 2019, Figure 3). The concept of circular cities captures this vision of the city as a complex system of resource flows and socially valuable uses (Williams 2021).

Implementing circularity within the built environment requires a systemic conceptualization that goes beyond materials, components, and structures to include whole buildings and neighborhoods.

This paper focuses on higher-level strategies of circularity by examining adaptive reuse as a key approach to promoting regenerative circularity in the urban built environment (Murray et al. 2017; Williams 2021). Unlike circularity solutions aimed at optimizing material resource flows, adaptive reuse also addresses the cultural, spatial, social, and ethical dimensions of sustainability. It challenges conventional notions of obsolescence and reveals the latent value of the existing built fabric (Wong 2023; Andreucci and Karagözler 2024). It not only reduces environmental impacts but also fosters equitable, resilient, and context-sensitive urban transformation.

Transformation in the built environment is not a new phenomenon. As Hermann Czech noted in Der Umbau (1996), urban life is inconceivable without continuous transformation. In recent years, the discourse on adaptive reuse has expanded from its origins in historic preservation (Arfa et al. 2022; Vafaie et al. 2023), through energy-efficient refurbishment (Richarz and Schulz 2011), to a broader recognition of existing buildings as valuable material resources. Today, adaptive reuse is increasingly seen not only as a conservation method but as a key approach to sustainable urban development and climate-responsive design. This shift reflects a growing awareness of the need to reduce grey emissions from demolition and new construction (Federal Foundation of Baukultur and Nagel 2022). A notable example of adaptive reuse as a transformation strategy is the German contribution to the 2012 Venice Architecture Biennale, Reduce / Reuse / Recycle (Petzet and Heilmeyer 2012). The work of the French architects Lacaton and Vassal (Lacaton et al. 2024) exemplifies this shift as well, with projects that prioritize minimal intervention, enhance living conditions, and create affordability, and social value. The emerging concept of Umbaukultur, a culture of transformation, has been explored in recent publications (Grafe and Rieniets 2020; Berke and de Monchaux 2023; Ngo et al. 2024), framing adaptive reuse as a response to broader societal megatrends (Naisbitt 1982). Another example is the project Obsolete Stadt (Rettich et al. 2023), which describes how buildings deemed obsolete can instead offer spatial potential for inclusive, community-oriented, climate-friendly, and coproductive development in growing cities. Such approaches emphasize “the potential of existing buildings to experiment with uses, programs, life models and economies that new buildings can no longer offer under today’s more restrictive financial, legal and construction conditions” (Ngo et al. 2024: 2). The experimental nature of transformation processes and the pioneering practices of temporary and interim use have been studied (Lange et al. 2007; Haydn and Tempel 2006; Oswalt et al. 2013) and continue to influence urban transformation initiatives. Recent research on multiple uses (Schröer 2019; Foerster and Teamwerk Architekten 2021) reflects a growing interest in multicoded spaces. This research overview and current discourse reveal a convergence of adaptive reuse and regenerative circularity. Instead of treating them as competing approaches, we seek to introduce an integrated concept to advance regenerative urban development.

To explore a concept of circularity that incorporates building use, user needs, and the urban context, we built on existing concepts and graphical models to derive the concept of circular uses. We revisited and expanded Stewart Brand’s layer model (1995) to include the use, zones of uses, and context of a building (see Figure 4). Visualizing the physical layers and spatial qualities of a building helps planners and designers understand the potentials of existing structures and develop transformation strategies during early planning phases.

In addition to the updated graphic model, this article proposes the concept of circular uses, which allows planners and designers to consider circularity in the built environment in a broader sense. The aim is to create a heightened awareness of the importance of use and to incorporate the concept in planning, design, and development processes at an early stage. We define the following types of circular building use: continued use, change of use, intensified use, and multiple use (see Figure 5).

In the case of continued use, buildings continue to be used without any change of use. Interventions in the fabric of the building may nevertheless be necessary for functional, energy-related, or aesthetic reasons. Change of use, in most cases, necessitates a more substantial conversion, as new functional and regulatory conditions apply. Intensified use describes a sufficiency approach with more use or users in the same space or the same use in less space. It also includes redensification as an intensified use of already built-up urban land. With multiple use, several uses overlap in terms of time and/or space; rooms are thus used differently depending on the time of day or changing requirements.

To explore and validate the concept of circular uses, we conducted a series of expert interviews. We carried out twenty-one semi-standardized interviews (Table 1). The selection of experts was based on the quadruple helix model, which includes academia, industry, government, and civil society (Carayannis and Campbell 2022). We have extended the model to include architects and urban planners, as they have an intersectional position and thus a special role in cocreation, process design, and project coordination. We interviewed experts and stakeholders in leading positions as they have a good overview of the processes, options for action, and hurdles. The interviews lasted between 30 and 90 minutes and covered the following topics: a) circularity and terms, b) buildings and their properties, c) developments, trends, and context, and d) processes and stakeholders (Table 2).

All interviews were recorded, transcribed, and thematically analyzed. We coded the transcriptions to identify similarities and opposing positions. Artificial intelligence tools were used to iteratively adjust the coding (Coral AI) and to support meaningful translations of the interview texts (Claude 3.5 Sonnet, ChatGPT-4, DeepL).

The concept of circular uses was well received by most interviewees. Some, however, initially questioned the necessity for a reframing of already existing practices. Urban planners emphasized the inherent circularity in city development, highlighting the historical precedent for continuous urban transformation (P1). Of the four types of circular uses discussed, change of use was central to most conversations. Continued use, intensified use, and multiple use expanded the discourse toward a broader range of higher-level circular strategies. Additional concepts of circular use were introduced and will be discussed below.

The interviews confirm that the implementation of circular uses in the built environment faces a range of interconnected challenges that span regulatory, economic, technical, and cultural domains (Table 3). Rigid building codes and outdated planning laws often prioritize new construction, creating legal and procedural barriers to adaptive reuse. The condition of existing structures, such as low ceiling heights, contamination, and missing documentation, further complicates transformation efforts. Financially, adaptive reuse is often perceived as riskier and less predictable than new builds, with supposedly higher costs associated with renovation and adaptation, limited incentives, and unfavorable tax structures. In parallel, a persistent cultural bias within the construction industry continues to favor demolition and new construction over adaptive reuse. Additionally, conventional planning processes rarely allow for the early, cross-disciplinary collaboration necessary to address the multifaceted demands of circular projects.

Successful strategies for implementing circular uses described by the interviewees focus on maximizing the potential of existing structures while minimizing waste and resource consumption. In addition to the circular uses that we proposed, the experts also mentioned minimal refurbishment, interim uses and mixed use of buildings and neighborhoods, uses that can generate both ecological and social value. These concepts can be assigned to the four proposed circular uses as they are expansions of these concepts (Table 4). Examples given by the interviewees included change of use projects such as converting offices into student housing with minimal alterations to the building’s structure. While strategies based on short term housing regulations provide immediate, often low cost solutions, challenges include managing legal regulations and ensuring long-term social justice. Temporary or interim uses allow for testing and experimentation, but users must often accept less-than-ideal conditions and precarious lease conditions. The term pioneering use indicates the intent of longer-term continuity of tested uses after a transitional process. Additionally, strategies like intensified or multiple use approaches aim to better utilize space, including accommodating evolving needs such as multigenerational living, yet require careful planning and management to address potential conflicts and ensure long-term sustainability.

Interviewees emphasized that a context-specific approach, grounded in local needs and community participation, is critical for revitalizing underused buildings and spaces
(Table 5).

Mixed-use developments and multifunctional designs were highlighted for their ability to support social, commercial, and cultural life, while also remaining adaptable over time. Moreover, enhancing sustainable infrastructure such as green spaces, public amenities, and public transportation networks not only improves environmental quality but also strengthens the viability and equity of transformation efforts. Developing transformation projects in contexts with existing sustainable energy and public transportation infrastructure supports whole lifecycle sustainability. Ultimately, aligning building reuse with the broader urban ecosystem enables socially and ecologically resilient neighborhood development.

Interviewees highlighted specific material and spatial qualities that make buildings or spaces particularly well-suited for circular uses by enhancing the durability and the ability to accommodate change and adapt to different uses over time. A detailed analysis of these
criteria will be published in a subsequent paper.

The interviews show a growing consensus that circular transformation in the built environment requires a fundamental rethinking of conventional planning and design processes (Table 6). Rather than adhering to linear, segmented workflows, practitioners advocate for iterative, whole lifecycle-oriented models that begin with early context analysis (phase zero) and extend through long-term building management and monitoring (phase ten). A key insight is the necessity of multidisciplinary collaboration and stakeholder engagement from the outset, ensuring that social, environmental, and economic considerations inform decision-making. Participation, communication and moderation are emphasized as essential for navigating complex urban dynamics, fostering trust, and reflecting diverse interests. The interviews also underscore the importance of mindset and institutional leadership in driving innovation, particularly within municipal structures. Additionally, evaluation methods based on multiple possible use scenarios are seen by some experts as more effective for guiding sustainable choices than lifecycle metrics based on a single use scenario. Together, these insights point toward an integrated planning and design approach that is context-sensitive, collaborative, and oriented toward long-term urban resilience.

The interviews underscore the urgent need for regulatory, administrative, and financial innovations to support a circular transformation (Table 7). Flexible, goal-oriented legal frameworks are essential to navigate the unique challenges of existing structures, replacing rigid, one-size-fits-all standards. The experts referred to current approaches like the Umbauordnung (BAK 2023) and the federally endorsed Gebäudetyp e (BMWSB 2024) that offer pathways to enable simplified, creative approaches to building reuse. Interviewees emphasized that building authorities, while constrained by current laws, do have interpretive leeway and must engage earlier and more proactively in the planning process. Economically, adaptive reuse faces hurdles from market-driven preferences for new construction, underlining the necessity of new financial models and incentives to de-risk and promote transformation efforts. Together, these insights point to a growing recognition that legal, administrative, and market structures must evolve to make circular and adaptive practices viable at scale.

A recurring tension lies in diverging perspectives between key actors: while architects, developers, and planners often feel constrained by rigid regulations, legal experts and city officials argue that existing frameworks already provide room for action, pointing instead to a lack of willingness or creativity in implementation. This disconnect signals the need for clearer communication, collaborative frameworks, and a collective reevaluation of how regulations are interpreted and applied.

A key area of consensus among interviewees was the value of flexibility and adaptability, both in building design and in the regulatory environment. Circular uses benefit from building designs that can be modified over time with minimal intervention. Strategies like change of use and multiple uses benefit from flexible conditions that encourage creative solutions without triggering costly structural changes or complex regulatory processes.The interviews highlight the need for regulatory reform and innovative financial mechanisms. Regulations must evolve to enable adaptive reuse and flexible approaches, reducing bureaucratic barriers and promoting long-term sustainability. Financial mechanisms, such as subsidies and tax incentives, are essential to encourage the regenerative transformation of existing buildings.

Participants also emphasized the importance of systemic and holistic approaches that integrate environmental, social, and economic dimensions across planning and design. Successful circular transformation depends on a whole lifecycle approach starting from the earliest stages – strategic thinking, analysis, evaluation, programming, and concept development – through iterative, feedback-driven implementation and finally use phase monitoring and evaluation. Community engagement emerged as another vital aspect. Thorough needs assessments, participatory design processes, and continuous feedback loops were seen as crucial, particularly for projects involving multiple or overlapping uses. Projects that align with community aspirations not only foster a sense of ownership and acceptance but also yield more meaningful and lasting transformations.

Additionally, professionals in real estate, city administration, architecture, and urban planning must shift their mindset to embrace circularity as a standard practice, fostering iterative processes and interdisciplinary collaboration. The complexity of circular transformation projects demands process-specific expertise beyond generic planning and design methods. Practical knowledge, targeted facilitation and participation, as well as a grounded understanding of legal and procedural contexts significantly increase the likelihood of successful implementation. To this end, we propose the institution of an Urban Transformation Office (UTO), aiming to bridge scales (building, neighborhood, city, region) and mediate between stakeholders, enhancing knowledge transfer and ensuring effective implementation across diverse contexts.

For the broader implementation of circular use strategies, we identified the following research fields. One major area of inquiry concerns the legal and regulatory frameworks and barriers that govern adaptive reuse. Evaluating how regulatory reforms, such as modifications to zoning laws and building codes, can support circular use practices and make adaptive reuse more feasible is also necessary.

In parallel, economic research is needed to evaluate how financial incentives, such as grants, tax credits, or low-interest financing, can encourage developers to prioritize long-term investment in existing buildings over demolition and short-term returns. Interdisciplinary development of new economic models that capture the long-term social and environmental benefits of circular uses will be crucial. This includes conducting whole lifecycle cost analyses to compare scenarios like adaptive reuse with demolition and new construction, incorporating environmental costs, embodied carbon, and future adaptability into the equation.

Another focus is the socioeconomic impact of transformation projects on local communities. Research should examine how circular strategies affect housing affordability, displacement, social equity, community resilience, and neighborhood identity. This entails investigating the role of key local actors, including municipal decision-makers, developers, and civil society groups, in shaping project outcomes. Additionally, investigating the relationship between community engagement and the economic success of projects could yield valuable insights into how to align community needs with broader economic objectives. Insights into how participatory planning processes affect design quality, maintenance, and long-term viability could strengthen the case for more inclusive approaches.

Developing new planning tools and methods that support circular approaches is also needed, especially for the early design phases. Approaches such as systems thinking, architectural programming, graphic analysis and representation, and scenario planning can help integrate circularity goals from the outset. Methodological frameworks that facilitate decision-making under conditions of uncertainty, while integrating environmental, social, and economic information, can enhance strategic planning. Additionally, establish-ing feedback mechanisms during building use and after project completion, such as postoccupancy evaluations, can inform future projects and ensure end-of-life reuse strategies.

Comparative case studies of successful adaptive reuse and neighborhood activation projects can provide insights into effective regulatory frameworks, financial models, building properties, and planning strategies that support circular transformations while ensuring safety, social equity, and material and spatial quality. Lastly, educational and training programs for architects, planners, and community stakeholders should be expanded to include circular design principles and interdisciplinary methods. Building professional capacity and enhancing public knowledge is essential for embedding circularity into standard urban planning and architectural practice.

The expert interviews show that circular use strategies – continued use, change of use, intensified use, and multiple use of existing buildings and spaces – represent a promising avenue for achieving more sustainable, resilient, and socially inclusive urban development. These strategies enable cities to reduce resource consumption, extend the life of the existing building stock, and adapt urban environments to changing needs without erasing their historical or cultural fabric.

Yet the successful implementation of circular use strategies depends on significant shifts – in mindset, regulation, economic frameworks, and design methodologies. It requires embracing complexity rather than avoiding it. Stakeholders must move beyond the conventional model of top-down planning and static regulation toward collaborative planning and design practices and flexible regulations.

One key insight from the interviews is that many of the tools and policies needed for this shift already exist in some form. What is lacking is not necessarily comprehensive legal reform, but institutional coordination, shared understanding, and the procedural knowledge required to activate the existing possibilities effectively. This highlights the urgent need for intermediary institutions that can translate between legal and economic frameworks, community aspirations, and architectural and planning practice.

Economic transformation is also essential. The current real estate market heavily favors new development and short-term returns. To counter this, policy-makers must introduce incentives that reward long-term thinking, environmental stewardship, and community-oriented design. Financial structures should reflect the true lifecycle costs of buildings, incorporating environmental externalities and social impacts.

Importantly, circular strategies must be embedded from the earliest planning stages. Early-phase programming, contextual research, and inclusive design processes are critical to identifying transformation potential and ensuring projects remain viable and responsive throughout their life cycle. These approaches need to be supported by adaptive policy, interdisciplinary expertise, and strong networks across public, private, and civic sectors. The development of new methodologies, educational programs, and collaborative networks is essential for fostering a culture of innovation and ensuring that circular urban transformation becomes a mainstream practice.

In sum, the transition toward regenerative urban development is not only possible, it is already underway in practice. However, scaling it requires concerted action. Cities must invest in regulatory reform, knowledge transfer, and capacity building. Architects and planners must lead with creativity, whole lifecycle and systems thinking, and community engagement. Real estate developers must take responsibility for the neighborhood context and prioritize long-term environmental and social outcomes over short-term profit. And public institutions must create the enabling environments that make circular strategies economically viable and socially equitable. If these elements come together, regenerative circularity can become a core principle of sustainable urban development, moving beyond material reuse and resource efficiency toward a resilient and equitable urban future.