The UK government’s Clean Growth Strategy unambiguously described the decarbonisation of heat as the UK’s greatest policy and technical challenge in meeting our carbon targets. Maximising the potential for energy efficiency in the existing domestic stock is critical to the low-carbon heat transition. Good information exists on the technical potential for energy efficiency measures in the UK stock, however, a lack of knowledge about current stock conditions and in-use factors places considerable uncertainty on how much of this technical potential is achievable in practice. \ud This study uses data from the fifth carbon budget (CB5) policy projections and updates the in-use factors using measured data from the National Energy Efficiency Database (NEED). This results in a 26% shortfall by 2035 in the anticipated energy savings through cavity, solid wall, and loft insulation compared to what is assumed in the CB5 projections. This will have costly implications for meeting future carbon budgets. Risks and policy implications are discussed. The practical potential for energy efficiency measures beyond cavity, solid wall, and loft insulation is explored.