RdSAP 10 is the latest version of the government’s method for rating existing homes, in use since 15 June 2025. The building has not changed, but what an assessor writes down about it has, which is why two certificates for the same home, one older and one newer, may not match.
What RdSAP is
The government rates the energy performance of homes with the Standard Assessment Procedure, SAP. For an existing home, a shorter version called reduced data SAP, or RdSAP, has been used since 2005. It works from a set of assumptions about the home, so an assessor collects less information than a full SAP calculation for a new build would need. Every EPC for an existing home comes out of it.
What assessors record now
RdSAP 10 asks for more, and more precise, information than the version before it. The main differences, as my accreditation scheme set them out:
| Windows | Every window is measured, with its frame type and a finer choice of when the glazing was fitted |
|---|---|
| Lighting | The bulbs are counted, not just the fittings, and LEDs are told apart |
| Ventilation | Open fireplaces, chimneys and flues are recorded, with more options for mechanical ventilation |
| Walls | Two wall types per part of the building, and the real thickness of wall insulation where it is known |
| Roofs and lofts | A wider range of insulation depths, two ways of assessing a room in the roof, and a recognised option for a shop or unheated space above a flat |
| Floors | Floor insulation added later can be recorded at its thickness |
| Hot water | The cylinder’s size counts, and its heat loss where documents show it |
| Newer technology | Solar battery storage, diverters and small hydro systems are recognised |
| Evidence | An air pressure test result can be used where there is a document for it |
Source: Elmhurst Energy, “SAP 10 analysis of changes”, 13 May 2025. Heated basements also gained more detail.
The recommendations changed too
The list of improvements on the certificate is generated from the same data. Under RdSAP 10, heat pumps are suggested more often for homes that are already well insulated, where the change is judged cost-effective, and battery storage or solar diverters only appear where solar panels are already fitted.
Why your new EPC may differ from your old one
- More is measured. Windows, bulbs and chimneys that an older assessment summarised are now counted one by one, and the extra detail can move the result up or down.
- Evidence counts for more. Insulation and new windows are only credited if they can be seen or there is paperwork, and the boiler’s efficiency rests on evidence too. Without it, the rating can come out lower than expected.
- It is not a like-for-like comparison. An older certificate was worked out by an earlier version of the method. A difference between the two is not on its own a sign that either is wrong.
If you think a new certificate has missed something, start with the evidence: the paperwork worth finding before a visit, and how to challenge an EPC afterwards.
Do you need a new EPC because of it?
No. An EPC is valid for ten years from the date it was lodged, and the change of method did not alter that. A certificate from 2020 can still be used for a sale, or for a new let if it meets the band E minimum, until it runs out. You might choose a new one anyway if you have made improvements since, because only a new assessment will record them.
The rules assessors follow
Alongside the method, assessors work to a set of conventions published by BRE that say how to record awkward cases consistently. They are updated from time to time; the current version, 12.2, is dated 6 August 2026. The current version of the method itself is RdSAP 10.0.1.
What comes next
RdSAP 10 is not the end of it. The government plans new-style EPCs built on a new calculation, the Home Energy Model, with four headline measures instead of one. In March 2026 it moved their launch to the second half of 2027, and in June 2026 it delayed the Home Energy Model itself. Landlords should note that the 2030 standard for rented homes is built around those new EPCs.
Common questions
What is RdSAP 10?
The version of the government’s reduced data method used to produce EPCs for existing homes since 15 June 2025. It records more detail than the version before it.
Did RdSAP 10 make EPC ratings go down?
Not as a rule. Some homes score higher and some lower. Ratings can come out lower where improvements cannot be evidenced, because only work that can be seen or proved is credited.
Is my old EPC still valid after RdSAP 10?
Yes. An EPC stays valid for ten years from the date it was lodged, whichever version of the method produced it.
What is the difference between SAP and RdSAP?
SAP is the full method, used for new homes. RdSAP is the reduced data version for existing homes, which works from assumptions so less has to be collected on site.
Where to go next
What happens during an EPC assessment, EPC ratings explained, and how to improve your rating.