This paper develops a proposal for a stepped approach to the final phase-out of new fossil fuel heating systems through a Carbon Intensity Standard (CIS) for future retrofit domestic heating. It proposes what the SEA believes are sensible, implementable, step changes in carbon intensity of future replacement heating systems, which progressively reduces the permitted carbon emissions from new installations or the replacement of major components of heating systems over the 2029 – 2036 period. This policy proposal sets out a long-term, technology agnostic, market-driven decarbonisation of domestic space and water heating. It avoids picking winners and would give investors the confidence to invest in the technologies and infrastructure that will take the future place of today’s fossil fuel boilers, against a stable and predictable regulatory background.