What is the difference between SAP and RdSAP?
SAP is the full Standard Assessment Procedure used for new build, calculated from design data. RdSAP is the reduced-data version used for existing homes, where an assessor records what is visible and the software fills gaps with age-and-construction defaults. Commercial buildings use SBEM instead.
- RdSAP defaults are deliberately conservative.
- Evidence overrides a default; assertion does not.
- SAP produces the certificate for a newly built dwelling.
- SBEM models commercial buildings zone by zone.
Three methods, three kinds of building
- SAP — the full Standard Assessment Procedure, used for new dwellings, calculated from design and as-built data.
- RdSAP — the reduced-data version, used for existing homes, where an assessor records what is visible.
- SBEM — the Simplified Building Energy Model, used for commercial and other non-domestic buildings.
Why RdSAP defaults matter so much
RdSAP exists because you cannot open up an occupied house to see what is inside its walls. Where the assessor cannot establish something, the method supplies a default based on the age and construction of the property — and those defaults are deliberately conservative.
That single design decision explains most of the gap between what owners expect and what the certificate says. An insulated cavity with no paperwork is recorded as an uninsulated cavity. A loft you cannot get into is recorded at the depth typical for the era. Neither is the assessor being difficult; it is how the method works.
What overrides a default
- Cavity-fill or loft-insulation certificates, including grant-scheme paperwork
- FENSA or equivalent glazing certificates
- Boiler installation and commissioning documents
- Building-control completion certificates for extensions
- Visible physical evidence — an accessible loft, a drilled-and-filled pattern on the brickwork
SBEM is a different exercise
Commercial buildings are not matched to archetypes. SBEM builds an explicit zone-by-zone model — activity type, floor area, construction, heating, cooling, ventilation and lighting for each zone. It takes considerably longer, which is why commercial EPCs are quoted per building rather than from a price list.
What this means practically
If you want an accurate rating, the paperwork matters more than anything you can do on the day. Find it before the assessor arrives. See our domestic EPC service or get a quote.
Looking for your own area? We have local EPC guides for Warwick city centre, Barford, Saltisford, Woodloes Park, Hampton Magna, Emscote, Wolverton and every other Warwick area.
