The UK’s net-zero by 2050 commitment necessitates urgent housing sector decarbonisation, as residential buildings contribute approximately 17% of national emissions. Post-1950 construction prioritised speed over efficiency, creating energy-deficient housing stock that challenges climate objectives. Current retrofit policies focus primarily on technological solutions—insulation and heating upgrades—while neglecting broader sustainability considerations. This research advocates systematically integrating Circular Economy (CE) principles into residential retrofit practices. CE approaches emphasise material circularity, waste minimisation, adaptive design, and a lifecycle assessment, delivering superior environmental and economic outcomes compared to conventional methods. The investigation employs mixed-methods research combining a systematic literature analysis, policy review, stakeholder engagement, and a retrofit implementation evaluation across diverse UK contexts. Key barriers identified include regulatory constraints, workforce capability gaps, and supply chain fragmentation, alongside critical transition enablers. An evidence-based decision-making framework emerges from this analysis, aligning retrofit interventions with CE principles. This framework guides policymakers, industry professionals, and researchers in the development of strategies that simultaneously improve energy-efficiency, maximise material reuse, reduce embodied emissions, and enhance environmental and economic sustainability. The findings advance a holistic, systems-oriented approach, positioning housing as a pivotal catalyst in the UK’s transition toward a circular, low-carbon built environment, moving beyond isolated technological fixes toward a comprehensive sustainability transformation.

Decarbonising the construction industry’s substantial ecological footprint demands credible substitutes that preserve structural performance while valorising waste. Although construction and demolition waste (CD&W) has been widely studied, the vast potential of agricultural residues (e.g., corncob, rice husk) and, crucially, their synergy remains underexplored. This study couples a systematic literature review with mathematical modelling to evaluate binary CD&W–agro-waste binders. A modified Andreasen–Andersen packing framework and pozzolanic activity indices inform multi-objective optimisation and Pareto analysis. The optimum identified is a 70:30 CD&W-to-agricultural ratio at 20% total cement replacement, predicted to retain 86.0% of OPC compressive strength versus a 79.4% average for single-waste systems (8.3% non-additive uplift). Life-cycle assessment (cradle-to-gate) shows a 20.3% carbon reduction for the synergistic blend (vs. 19.6% CD&W-only; 19.3% agro-only); when normalised by strength (kg CO₂-eq/MPa·m³), the blend delivers 6.3% better carbon efficiency than OPC (5.63 vs. 6.01), outperforming agro-only (5.79) and CD&W-only (6.61). Global diversion arithmetic indicates feasible redirection of 0.246 Gt y⁻¹ of wastes (5.7% of CD&W and 1.8% of agricultural residues) at 30% market penetration. Mechanistically, synergy arises from particle size complementarity, complementary Ca–Si reactivity generating additional C–S–H, and improved rheology at equivalent flow. Monte Carlo analysis yields a 91.2% probability of ≥40 MPa and 78.3% probability of ≥80% strength retention for the optimum; the 95% interval is 39.5–55.3 MPa. Variance-based sensitivity attributes 38.9% of output variance to the Bolomey constant and 44% to pozzolanic indices; interactions contribute 19.5%, justifying global (not local) uncertainty propagation. While promising, claims are bounded by cradle-to-gate scope and the absence of empirical durability and end-of-life evidence. The results nevertheless outline a tractable pathway to circular, lower-carbon concretes using co-processed waste. The approach directly supports circular economy goals and scalable regional deployment.

The United Kingdom's legislated commitment to net-zero carbon emissions by 2050 necessitates urgent decarbonisation across all economic sectors. The residential housing sector presents a particularly significant opportunity, accounting for approximately 20% of national greenhouse gas emissions. A substantial portion of the UK housing stock comprises post-1950 dwellings constructed during periods of rapid expansion, when resource efficiency and thermal performance were subordinated to addressing immediate housing shortages. These buildings emerged in an era characterised by perceived energy abundance and limited awareness of environmental consequences, resulting in unanticipated energy performance deficiencies that now challenge contemporary climate objectives. While energy-efficient retrofitting has become integral to national decarbonisation policy, current initiatives predominantly emphasise technological interventions—such as thermal insulation, fenestration improvements, and heating system upgrades—often without adequately addressing broader sustainability imperatives or natural resource stewardship. This research proposes a paradigmatic reorientation through the systematic integration of Circular Economy (CE) principles into residential retrofit practices. CE approaches—characterised by material circularity, waste minimisation, adaptive design strategies, and comprehensive lifecycle assessment—offer enhanced environmental sustainability and economic resilience compared to conventional retrofit methodologies. The investigation employs a multi-methodological approach encompassing systematic literature analysis, comprehensive policy review, stakeholder engagement, and critical evaluation of retrofit implementation across diverse UK contexts. This research identifies significant barriers to CE integration, including regulatory constraints, workforce capability gaps, and supply chain fragmentation, while also recognising potential enablers for transition. Building upon this analysis, the study develops an evidence-based decision-making framework that systematically aligns retrofit interventions with CE principles. This integrative framework is designed to inform policymakers, industry practitioners, and researchers in conceptualising and implementing retrofit strategies that simultaneously enhance energy performance, optimise material circularity, minimise embodied carbon, and strengthen long-term environmental and economic resilience. The findings advance a holistic, systems-oriented approach to residential retrofitting that positions the housing sector as a critical catalyst in the UK's transition toward a circular, low-carbon built environment.