Why Your EPC Score May Be Wrong: 'Assumed' Values Explained

If your rental property's EPC sits at a D, there is a fair chance the certificate is underselling the building. The reason is one word that appears on thousands of EPCs: assumed. Wherever it appears, the assessor did not measure anything. The software filled in a worst-case default for the property's age, and your score carries the penalty.

This matters because every private rented home in England and Wales needs an EPC C by 1 October 2030, and most landlords assume the only way up is spending on insulation, glazing or a new heating system. For a specific group of D-rated properties there is a cheaper first step: measure what was assumed.

Why do EPCs contain assumed values at all?

A standard EPC assessment is a visual survey. The assessor records what they can see and verify: boiler make, loft insulation depth, glazing type. Anything they cannot see, such as insulation inside a wall or how much air leaks through the fabric, gets a default from standard tables based on the year the property was built.

Defaults are cautious by design. A 1930s terrace is assumed to leak like a typical unimproved 1930s terrace, even if it has been re-pointed, re-floored, draught-proofed and fitted with tight modern windows over the decades. The certificate scores the assumption, not the building.

How to check your own certificate in two minutes

Look up your property on the government's Find an Energy Certificate service and open the full certificate. Scan the property features table for the words "assumed" or "not recorded". Each instance is a place where your score may be below the building's real performance. Air tightness is the big one: it is part of the SAP calculation behind every EPC, and it is almost never measured on existing homes.

What changed with RdSAP 10

RdSAP 10 became the mandatory methodology for existing-home EPCs on 15 June 2025, and it explicitly permits a verified air permeability test result to replace the assumed default. That is the mechanism behind Thermova's guaranteed EPC C service for landlords: an accredited test measures how airtight the property actually is, a qualified Domestic Energy Assessor re-runs the assessment with the measured figure, and the corrected EPC is lodged on the national register, valid for ten years.

No building work. No tenant disruption. The test takes around two to three hours, uses Pulse equipment rather than a doorway fan, and tenants can stay in the property throughout.

Which properties benefit most?

The honest answer is: not all of them. If a property leaks as much as the default assumes, measuring will not help, which is why a no-result-no-charge model only works with pre-screening. In practice the strongest candidates share three traits:

• A current score in the mid-to-high D band. Certificates between D63 and D68 are typically close enough for measured evidence alone to cross the C threshold at 69.
• Several "assumed" entries on the certificate, especially for air tightness, walls or floors.
• A well-maintained older building. Solid, cared-for pre-1980s housing stock is regularly tighter than its age-band default.

Properties further down the scale usually need more than measurement. If yours is an E, F or G, the route to a C combines low-cost fabric work with grant-funded improvements, and it is worth reading our guide to what each route to a C actually costs before spending anything.

Why the window for this route is closing

The Home Energy Model is scheduled to replace RdSAP in the second half of 2027, and under the reformed methodology assumed values are being scrapped in favour of full physical evidence. A C secured now under RdSAP 10 remains valid for its full ten-year term. Waiting means assessing under a stricter regime, closer to the 2030 deadline, alongside every other landlord who waited. We cover the change in detail in our Home Energy Model explainer.

Frequently asked questions