The Home Energy Model (HEM) is the UK government's replacement for the Standard Assessment Procedure (SAP) — the methodology used to assess the energy performance of homes for Building Regulations compliance and Energy Performance Certificates (EPCs). HEM has been in development since 2020 and began replacing SAP for new homes from 2024.
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SAP was designed in the 1990s and has been updated incrementally since. Its key limitation is a lack of granularity — it models average household behaviour rather than actual occupancy and usage patterns. HEM uses more detailed modelling of heat losses, solar gains, occupancy, and appliance usage, and is designed to be compatible with the Future Homes Standard and emerging technologies like heat pumps and smart controls.
What HEM means for new builds
New dwellings are now assessed against HEM to demonstrate compliance with Part L of the Building Regulations. The HEM calculation considers the entire energy balance of the dwelling, including the heating system, hot water, lighting, cooling, and renewables. For developers and self-builders, this means updated energy calculations from SAP assessors who have been trained in the new methodology.
The Home Energy Model is designed to provide a more accurate and technology-neutral assessment of home energy performance — one that reflects how homes actually perform, not just how they are designed to perform.
— UK Government, Department for Energy Security and Net Zero
Implications for EPCs
The government has also been consulting on reforming the EPC (Energy Performance Certificate) system alongside HEM. Current EPCs are widely criticised for being based on running costs (which favoured gas in a period of cheap gas) rather than carbon or actual energy performance. Reformed EPCs under HEM may include primary energy, carbon, and fabric efficiency metrics — giving homeowners, buyers, and lenders a more complete picture.
For homeowners carrying out renovations
For most homeowners renovating an existing property, HEM does not directly affect you unless you are building a substantial extension that triggers a SAP/HEM assessment as part of Building Regulations compliance. For extensions over a certain size, your builder or architect may need to commission an energy assessment to demonstrate Part L compliance — check with your local building control authority on the threshold that applies.