RdSAP 10: the quiet overhaul of the domestic EPC

RdSAP — Reduced Data SAP — is the methodology behind every existing-home EPC in the UK. Version 10, phased in from mid-2025, is its biggest revision since EPCs began, and it changes what surveys involve, what evidence counts, and how some properties score.

Why the methodology needed surgery

Old RdSAP was built for speed: where data was hard to collect, the methodology substituted assumptions keyed to property age. A 1900s terrace got 1900s default walls whether or not someone had insulated them; heat pumps were handled awkwardly by a model designed around gas boilers; solar and batteries were afterthoughts. The result was certificates that were cheap to produce and frequently wrong in the specifics — tolerable when EPCs were paperwork, less so now that MEES compliance, green mortgage pricing and grant eligibility all hang off the band.

What actually changed

Measured data replaces assumptions. More dimensions, more construction observation, less age-based defaulting. The survey interrogates the building rather than its birth certificate.
Mandatory photographic evidence. Everything material to the rating gets photographed into the assessment file — boilers, insulation, glazing, meters. Certificates became auditable artefacts.
Address-level geolocation. Orientation and location now feed the model properly, mattering most for solar calculations.
Fair treatment of low-carbon technology. Heat pumps, PV, batteries and modern controls are modelled substantially better. Homes with them frequently score higher than their old certificates did.
Tighter conventions for extensions and room-in-roof spaces — historically the sloppiest corner of domestic assessment, now measured and recorded separately.

What it means for you, by situation

Selling: if your current EPC is old and your home has undocumented improvements, a re-assessment under RdSAP 10 — with the paperwork from the preparation guide in hand — may produce a genuinely better band to market with. If your certificate is recent and decent, keep it; validity is ten years.

Letting: precision cuts both ways at the MEES boundary. A D that rested on generous defaults can re-assess to an E, or an E to a D. Re-assess when you control the timing, not when a tenancy renewal forces it — and use the evidence rules to your advantage, because documented improvements now bank reliably.

Heat pump, solar or battery owners: you are the clearest winners. If your certificate predates your installation — or predates RdSAP 10's fairer modelling — a re-assessment will likely recognise hardware the old methodology shortchanged. Installers have caught on too — Energy Concerns' Leicester team now suggest an updated certificate after every domestic PV fit. With green mortgage products pricing off the band, that recognition can be worth real money.

Buying: treat pre-2025 certificates on listings as approximations. The assessment walkthrough shows what a current survey covers; commissioning one post-purchase gives you the accurate baseline for any renovation plan.

The wider reform picture

RdSAP 10 is the delivered half of a broader EPC reform programme. Government has also consulted on redesigning the certificate itself — multiple metrics (fabric, heating system, smart readiness) instead of one cost-based score, and possibly shorter validity periods. None of that is law in 2026. What exists today is a sharper measuring instrument and unchanged legal thresholds; this page tracks the former, and our FAQs keep the legal facts straight, including the persistent myth that EPC C is already mandatory for rentals.

RdSAP 10 questions

Is my old EPC still valid now RdSAP 10 is in use?

Yes — certificates remain valid for their full ten years regardless of methodology changes. Nothing forces a re-assessment. The question only becomes live at the next sale, letting or re-assessment, when the property will be surveyed under the current methodology and may score differently than the old certificate suggested.

Will my rating go up or down under RdSAP 10?

It depends on what the old assessment assumed. Homes whose previous certificates leaned on pessimistic defaults — undocumented insulation, unrecorded heating upgrades — often improve when properly measured. Homes that benefited from generous old assumptions can slip. Heat pump, solar and battery owners generally do better, as the new methodology models low-carbon technology far more fairly.

Why does the survey take longer now?

More measurement and mandatory evidence. Assessors record more dimensions, more construction detail and photographic proof of what they enter. The 20–40 extra minutes is the price of certificates that are auditable and disputable on evidence rather than memory — and of fewer of the nonsense recommendations the old defaults generated.

Does RdSAP 10 change MEES or any legal thresholds?

No. The legal framework is unchanged: EPC E remains the minimum for rental property, and proposals to raise it remain proposals. What changes is measurement accuracy — which can move individual properties across existing thresholds in either direction when re-assessed. Landlords near the E boundary should re-assess on their own schedule rather than a tenancy deadline's.