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The built environment sits at the heart of the climate challenge UNEP and the Global Alliance for Buildings and Construction report that buildings account for around 21% of global greenhouse gas emissions (UNEP and GlobalABC, 2024). In 2022, the sector consumed 34% of global energy and produced 37% of energy-related CO₂ emissions. Despite decades of policy initiatives, rating systems, and green building strategies, these figures have changed little. The Paris Agreement set targets we all know by now: stay well below 2°C, aim for 1.5°C (UNFCCC, 2015). However, achieving these targets requires more than incremental improvements; it demands transformative changes across construction, operation, and end-of-life management of the built environment (UNFCCC, 2015).A critical yet underexamined reality shapes the scope of this challenge. Most buildings that will exist in 2050 are already standing. While global urban expansion in cities such as Lagos, Dhaka, and Mumbai will add substantial new building stock, mature cities in Europe and North America face a different situation. Approximately eighty percent of Europe's building stock projected for 2050 has already been constructed (McKinsey, 2021;World Economic Forum, 2022).Consequently, even if all new buildings were designed to meet net-zero targets, the sector would still fall far short of the emissions reductions required. The real imperative lies in retrofitting the existing building stock to achieve both energy efficiency and material circularity.Retrofitting presents its own complexities. Energy savings through improved insulation or highperformance glazing are well documented, yet less attention is paid to the sourcing and disposal of construction materials. Construction and demolition waste constitutes the largest waste stream in the European Union, reaching 374 million tonnes in 2016, excluding excavated soil (European Environment Agency, 2020). Globally, the construction sector consumes around half of all extracted raw materials and generates approximately one-third of all waste (Benachio et al., 2020). Although European Union directives set a seventy percent recovery target, much of the material counted as recovered is either crushed for road base or used for backfilling, practices that do not preserve functional or economic value (EEA, 2020;Moschen-Schimek et al., 2023).True circularity, where materials retain their original performance and are reincorporated into new projects, remains rare.Forum estimate that the retrofit market will grow from US$500 billion today to US$3.9 trillion by 2050 (World Economic Forum, 2024). Current annual retrofit rates remain below one percent of building stock. To align with net-zero goals, these rates must increase to three percent by 2030 and four percent by 2050. So, we face a choice. This special issue of Frontiers in Built Environment was conceived to address this gap. We sought papers that examine circularity in building retrofitting from multiple angles. Five were accepted. Before discussing them, it is worth setting out the research gaps that motivated this collection.The call for papers was informed by five interrelated gaps that continue to constrain the development of circular construction research.The first gap concerns scale. Much of the research on circular construction focuses on either individual buildings or city-wide systems, leaving the meso-scale largely unexplored. This intermediate level, involving coordinated groups of renovation projects, offers significant potential for optimizing material flows, reducing logistical inefficiencies, and justifying investments in recovery infrastructure that no single project could support. Jensen et al. (2018) identified this gap several years ago, and it remains largely unaddressed. The meso-scale is where circularity could move from isolated successes to systemic impact, yet empirical studies and practical guidance remain rare.The second gap is the persistent disconnection between theory and practice. Academic literature is replete with conceptual frameworks for circular construction, but actionable guidance for practitioners is scarce. Contractors seeking to salvage façade panels from a 1970s office building often find few usable methods or documented case studies. Practitioners require checklists, decision-making tools, and detailed examples that demonstrate how circular principles can be operationalized (Jensen et al., 2017). Without bridging this divide, circular construction will remain aspirational rather than implementable.The third gap is geographic. The evidence base for circular construction is concentrated in Western Europe and North America. Life cycle inventories, regulatory analyses, and empirical case studies from Africa, South Asia, and Latin America are limited. This is problematic because the majority of global floor area growth to 2030 will occur in countries that currently lack comprehensive building regulations (UNEP, 2024). Applying European or North American data to contexts such as a Sri Lankan ceramic tile factory or a Nigerian housing project risks misrepresentation and may lead to interventions that are technically sound but practically inappropriate. Developing region-specific evidence is therefore critical for global applicability.The fourth gap is disciplinary fragmentation. Scholarship relevant to building reuse and circular retrofitting is dispersed across adaptive reuse, heritage conservation, urban regeneration, and circular economy literatures. Each field publishes in separate journals, attends distinct conferences, and frames challenges in different ways. As a result, integrated understanding is limited, and potential synergies between insights from different disciplines remain underexplored. A more coherent synthesis across these literatures is needed to guide both research and practice.The fifth gap is conceptual conservatism. Construction management research has often relied on familiar frameworks and incremental approaches. Fresh perspectives from biomimicry, industrial ecology, or complexity science, which could offer novel insights into material flows and urban systems, remain underrepresented. Incorporating these perspectives could help identify new leverage points, stimulate innovative practice, and support systemic change.Five papers were accepted following peer review. Each tackles different aspects of circularity in retrofitting, employing distinct methods and operating at different scales. Ashrafi et al. (2025) tackle the meso-scale gap head-on. They reviewed 121 publications and developed a framework with three stages: Planning, Assessing, and Routing. The core idea is simple but underexplored: treat multiple renovation projects as a system, not as isolated jobs.Which projects will produce surplus materials? Which will need them? How do you coordinate timing and logistics so that supply meets demand?They tested this thinking on six quay wall and bridge renovations in Amsterdam, all managed by the same municipal authority. The results were instructive. Capstones removed from the Herengracht quay wall went directly to Prinsengracht, where they were cleaned and reinstalled.Concrete from Prinsengracht was processed and used to manufacture prefabricated elements for the Kloveniersbrugwal project. Timber piles from Jacob Catskade, no longer needed for foundations, found second lives in interior design applications and ecological bank protection. The result is thirty practical "levers" organised into ten building blocks. What distinguishes this paper is its orientation toward action. These are not principles derived from theory but specific moves that real project teams made when confronted with actual problems. The accompanying project sheets, which document timelines, actors, and decisions for each case, constitute a valuable resource in themselves. A contractor wondering how others have navigated facade reuse can find concrete examples here. Vijerathne et al. (2025a) contribute something different: life cycle assessment data from Sri Lanka. They evaluated ceramic tile manufacturing under four scenarios, replacing virgin materials with fly ash, recovering energy from biomass, switching to solar power, and combining all three. The integrated scenario reduced global warming potential by 21%, terrestrial acidification by 27%, and ozone depletion by 23% compared to conventional production. These are meaningful improvements, though not revolutionary. Perhaps the paper's greater contribution is simply producing Sri Lankan inventory data, filling a gap that limits the applicability of LCA in developing country contexts. Ghoz (2025) attempts the synthesis that has been missing from the literature. Through systematic review and thematic analysis of 28 papers spanning adaptive reuse, heritage conservation, urban regeneration, and circular economy literatures, she identifies 75 distinct challenges to residential building reuse. These are grouped into ten themes: economic viability, building conditions, design and technical issues, location, decision-making, policy and regulation, knowledge and skills, culture and awareness, community dynamics, and timeline pressures.Economic and financial constraints loom largest in this taxonomy, but what makes the analysis valuable is its attention to interconnections. A building in poor physical condition is not just a technical problem; it is a financing problem, because renovation costs become difficult to predict. Regulatory delays are not merely bureaucratic frustrations; they create financial risk that deters private investors. Skills shortages do not just slow projects down; they drive up labour costs and make business cases harder to close. The paper's contribution lies in mapping these interconnections systematically, providing a framework that researchers and policymakers can use to identify leverage points for intervention. Rahubadda et al. (2025) take a different path entirely, drawing on biomimicry to reimagine urban energy systems. Their inspiration is the mycorrhizal network, the underground fungal web through which forest trees share nutrients and chemical signals. Ecologists have documented how mature trees support struggling seedlings and how trees under stress receive resources from healthier neighbours. The so-called "Wood Wide Web" functions as a kind of cooperative infrastructure for forest ecosystems.Could buildings do something similar? The paper asks whether urban structures might share energy in analogous ways, with those generating surplus supporting those in deficit. The authors develop this idea conceptually, proposing a framework for adaptive energy sharing in carbonneutral cities. There are no simulations here, no pilot data, just the conceptual groundwork. Some readers may find this too speculative for a research journal. Others will welcome it as exactly the kind of fresh thinking that construction management scholarship tends to lack. We incline toward the latter view, while acknowledging that the real test will come when someone tries to implement these ideas.Reading these contributions collectively reveals several patterns that warrant careful scrutiny.One striking observation is that none of the papers evaluates the actual environmental performance of completed circular retrofits. There are no before-and-after measurements, nor is there systematic tracking of how much material is truly reused versus ultimately landfilled.Instead, the papers operate upstream, developing frameworks, guidance, taxonomies, and conceptual models. This focus likely reflects the emergent state of the field. Circular retrofitting remains sufficiently novel that the intellectual infrastructure for comprehensive evaluation is still under construction. Empirical performance data will inevitably emerge in due course, yet the absence of such evidence highlights a critical gap that the research community must address.In terms of addressing the gaps identified in our call for papers, the contributions perform moderately well, albeit unevenly. Ashrafi et al. (2025) Three cross-cutting themes emerge from the analysis. First, individual projects alone cannot achieve circularity. The meso-scale coordination documented by Ashrafi et al. (2025), the stakeholder alignment highlighted by Van Vooren et al. (2025), and the interconnected challenges mapped by Ghoz (2025) converge on the same conclusion: circular construction requires systemic change. Achieving this entails new forms of collaboration, structured information sharing across project boundaries, and business models capable of capturing value from material reuse. Previous research has suggested this necessity (Jensen et al., 2018), but these papers provide specificity and concrete illustrations.Second, the field lacks the knowledge infrastructure necessary to support circular construction at scale. Life cycle databases require regional granularity, materials must be tracked through successive building lifecycles, and supply and demand for secondary materials must be coordinated through dedicated platforms. Practitioner knowledge, while extant in fragments, remains uncodified and dispersed. The absence of an integrated system constrains both research and practice, limiting the capacity to translate circularity from concept to operational reality. Demand-side perspectives are also underrepresented. The papers concentrate almost exclusively on supply-side actors, including architects, engineers, contractors, material suppliers, and policymakers. Building owners and occupants, who ultimately determine whether circular approaches are adopted, are largely invisible. Questions of ownership, perception, and behavioural response remain unexamined: why do some owners embrace circular retrofitting while others abstain? What information or incentives might shift decision-making? How are reclaimed materials perceived by occupants in terms of quality, safety, and aesthetics? Understanding these dimensions, and developing strategies to stimulate demand, constitutes an important frontier for research.Collectively, these observations underscore that while the special issue advances conceptual and methodological frontiers, significant work remains. Bridging the divide between upstream frameworks and downstream performance, integrating social and economic dimensions, and engaging demand-side perspectives will be essential to establish a holistic understanding of circular retrofitting and to enable its widespread adoption.The five papers in this collection advance the research agenda on circularity in building retrofitting. They offer meso-scale frameworks, practical guidance, developing-country evidence, multidisciplinary synthesis, and conceptual innovation drawn from ecology. These are genuine contributions that fill gaps we identified in issuing the call for papers. We should be honest about limitations. The papers focus on frameworks and concepts rather than measured outcomes. None tracks what actually happens to materials in completed circular retrofits. Social sustainability questions go unasked. Demand-side perspectives, the views of building owners and occupants, are largely absent. These gaps mark territory for future research. The larger challenge remains daunting. If the World Economic Forum's projections are right, retrofit rates need to triple or quadruple over the next 25 years. Hundreds of millions of buildings will be renovated. Billions of tonnes of materials will be consumed. Whether this transformation embeds circular principles or replicates the wasteful patterns of conventional construction is genuinely uncertain. The papers in this collection help illuminate the path forward, but the research community, ourselves included, has considerable work ahead.