The EPC is the certificate: the public document with the A to G chart, lodged on the register, valid for 10 years. SAP, the Standard Assessment Procedure, is the government's methodology for calculating the energy performance score that decides the band GOV.UK SAP. BRE maintains SAP for government BRE. You never buy a SAP; you buy an assessment that runs one.
| Methodology | Used for | Data source |
|---|---|---|
| Full SAP | New builds; see new-build EPCs | Complete design specification: drawings, U-values, services |
| RdSAP (reduced data SAP) | Existing homes: every survey-based EPC | Assessor's on-site survey plus documented evidence, with standard defaults filling gaps |
What a "SAP rating" is
The number behind your band: an index of modelled running cost for heating, hot water and lighting under standardised occupancy, where higher is cheaper GOV.UK SAP. Band boundaries slice the scale: C starts at 69, D at 55, the floor of E at 39. The full mapping is on EPC ratings explained.
Why the distinction bites
Because the methodology has versions, and versions change scores. RdSAP moved to version 10 in June 2025 with evidence-based inputs; the same house can score differently under RdSAP 9 and RdSAP 10 without a brick changing. That story is on the RdSAP 10 page and the version comparison. And the whole SAP family is scheduled for replacement by the Home Energy Model, which is why long-term plans should not be anchored to today's arithmetic.