Ambitiously, the UK aims to cut greenhouse gas emissions by 80 per cent by 2050. Since the use of housing accounts for about 27 per cent of UK CO2, and most newbuild adds to the number of homes rather than substituting for them, housing’s biggest contribution to better energy use and lower carbon emissions in the UK will come from retrofitting the country’s existing stock. Moreover retrofitting particularly matters to registered providers of social housing, who seek guidance about the energy efficiency of their properties. This paper argues that an exclusive focus on just one of the technical, economic or social aspects of retrofit is inadequate. Using both theory and case-based experience, it discusses a number of ways, both technical and qualitative, of best measuring what retrofitting can do. It concludes that an integrated, comprehensive understanding of the retrofit process is essential to the making of informed decisions on the energy efficiency of homes, particularly at the scales required.