What happens in an energy assessment

Knowing what the assessor will do — and what they are legally allowed to assume when they cannot see something — takes the mystery out of the visit and usually improves the result. Here is the domestic assessment end to end.

Before the visit

The assessor confirms the address and any access constraints. Your job is evidence-gathering, covered fully in the preparation guide: boiler manuals, insulation receipts, glazing invoices, FENSA or building-control certificates. The principle that governs everything: assessors record what they can see or what you can document. Improvements hidden behind plasterboard without paperwork legally do not exist for rating purposes.

On the day: the survey, room by room

A typical home survey under RdSAP 10 runs 45 minutes to two hours depending on size and complexity. The assessor will:

• Measure the property — external or internal dimensions of every heated space, now with more rigour than the pre-2025 routine. Extensions and conversions are measured separately because they usually have different construction.
• Identify wall, roof and floor construction — cavity or solid walls, insulation evidence, loft depth measured by eye and tape. Expect the loft hatch to be opened.
• Record heating and hot water — boiler make and model (photographed), controls, thermostats, cylinder insulation, any heat pumps or secondary heating. RdSAP 10 treats low-carbon heating far better than the old methodology did.
• Count and classify glazing — single, double, triple; frame types; approximate ages.
• Log renewables and storage — solar PV, batteries, solar thermal, with capacities where documented. Another area RdSAP 10 substantially improved.
• Photograph everything — mandatory evidence requirements mean the file behind your certificate now contains dozens of images, which is what makes modern EPCs auditable and disputable in a way old ones were not.

The assessor does not need the property tidy, empty or warm. They do need access to every room, the loft hatch, the boiler and any meters — locked rooms get defaulted, and defaults never help.

After the visit: calculation and lodgement

The survey data goes into RdSAP software, which models annual energy demand against standardised occupancy — the same assumptions for everyone, so certificates compare like for like. Out comes the A–G band, the numerical score, and the recommendations list with indicative costs and savings. The assessor lodges the certificate on the national register, usually within 48 hours of the survey, where it is publicly checkable via gov.uk's find-an-energy-certificate service. It is valid for ten years.

If you disagree with the result

First conversation: the assessor. Most disputes are evidence gaps — the insulation you mentioned but could not document, the new boiler whose model went unrecorded — and a re-run with documentation is quick. Beyond that, every accreditation scheme operates a complaints process that audits the certificate against its photographic evidence file. Genuine errors get corrected and re-lodged without a fresh fee. What does not work is shopping for a second assessor and hoping — certificates are public, and conflicting lodgements invite scrutiny.

Commercial surveys: the same logic, scaled

Non-domestic assessments follow the same evidence-first principle with bigger machinery: zoning by activity, plant room inspection, SBEM or dynamic simulation instead of RdSAP. Timelines run two to four working days from survey to lodgement for most buildings. The commercial assessment page covers levels and pricing; costs for domestic work are on the EPC cost page.