Optimal decarbonization strategies for existing buildings: Comparative analysis of retrofitting, repurposing and replacement

Decarbonizing existing buildings is a critical priority for addressing climate change mitigation, but decision-making is complicated by the range of available competing intervention strategies: retrofitting, repurposing, or replacement through new construction. Each option has distinct implications for operational and embodied carbon, as well as overall costs. However, stakeholders currently lack a harmonized framework to compare these alternatives on a consistent carbon and cost basis—particularly when demolition is involved. Demolishing a building not only releases the embodied carbon stored in its materials but also requires new materials for reconstruction, compounding the overall carbon impact. Accurately accounting for these effects is essential, yet often overlooked or inconsistently treated in current approaches. Existing studies rarely offer side-by-side analyses or use compatible accounting methods, often leading to conflicting recommendations.This study develops an optimization framework that integrates building energy simulation with novel carbon accounting approaches. The framework explicitly includes the embodied emissions of both demolished structures and new interventions and introduces the concepts of “residual” and “salvage” carbon to account for building components and technologies with remaining useful life before and after the planning horizon. By doing so, the model enables a direct comparison of the optimal intervention measures selected across retrofitting, repurposing, and replacement strategies over a long-term investment horizon.Results show that retrofitting is typically the most carbon- and cost-effective strategy, outper forming repurposing and replacement in most scenarios. Incorporating salvage carbon encourages major retrofits to be phased over time, reducing the need for large upfront investments. Accounting for the remaining useful carbon of existing assets allows stakeholders to fully capture the impacts of demolition and replacement.