The Standard Assessment Procedure (SAP) has been the UK's domestic energy compliance methodology since 1992. Part L 2026 retires it for new dwellings, replacing it with the Home Energy Model. This page explains how HEM differs and why it matters for designers.
Why SAP is being replaced
SAP runs on monthly steady-state assumptions and does not realistically model self-consumption, batteries, EV charging or smart heating controls. As renewable technologies became central to compliance, SAP's limitations became significant. HEM resolves them by running half-hourly dynamic simulation over a full year.
How HEM differs
HEM models the dwelling 17,520 times per year (every 30 minutes). It tracks solar generation against demand profile, battery state of charge, EV charging schedules and ASHP COP variation with outdoor temperature. The output is more realistic — and rewards the design choices (battery, smart controls) that actually deliver bill savings.
Assessment time impact
Typical SAP assessment time on a 3-bed house: ~20 minutes. Typical HEM assessment: ~1 hour 40 minutes. SAP/HEM consultancies are recruiting and retraining; expect HEM modelling charges to be 2.5–3× legacy SAP for the first 18 months of the transition.
Software status
Elmhurst, Stroma, BRE and NHER have HEM implementations either released or in late beta as of mid-2026. The Government's reference HEM toolset is also available for verification. Practices need a HEM-capable workflow before March 2027.
Transitional rules for assessors
During the 24 March 2027 – 24 March 2028 transitional period, projects qualifying for transitional protection may use SAP 10.3. After that, HEM is the only valid compliance route. Energy Performance Certificates continue to be issued through HEM outputs.