What assessors look for: turning work into points
The most expensive mistake in the D-to-C process is not a bad measure — it is good work the assessor cannot record. RdSAP runs on what can be observed or evidenced. Hidden insulation with no paperwork is, for scoring purposes, insulation that does not exist.
How an RdSAP assessment works
A domestic energy assessor spends 45 minutes to an hour in the property measuring and recording: construction type and wall thickness, insulation where visible, glazing, the heating system and its controls, hot water arrangements, lighting, and — since RdSAP 10 arrived in 2025 — actual window dimensions, ventilation details and any PV or battery systems. The software applies standard assumptions for anything unobserved. The assessor cannot take your word for what is behind the plasterboard: conventions require observation or acceptable documentary evidence, and the lodged certificate is audited against those rules.
The evidence file, measure by measure
| Measure | What the assessor can use |
|---|---|
| Cavity wall insulation | CIGA or installer guarantee certificate; drill-pattern photos; building control notice. The guarantee is the gold standard — file it with the deeds. |
| Loft insulation (boarded over) | Dated photos taken during installation showing depth against a tape measure; supplier invoice stating thickness; or visible depth at the hatch. |
| Solid wall insulation | Installer certification (e.g. under PAS 2030), building control completion, before/during photos. Internal insulation is invisible when done — paperwork is everything. |
| New boiler | Make and model visible on the unit (the assessor looks it up in the product database); Gas Safe / Benchmark commissioning record helps date it. |
| Heating controls | Observed directly — make sure the room stat is on the wall and TRV heads are fitted, not in a drawer awaiting decoration. |
| Solar PV / battery | MCS certificate, commissioning documents, generation meter. RdSAP 10 records system details more fully than before — have the kWp figure to hand. |
| Floor insulation | Photos during the work plus invoices — once the boards are down, observation is impossible. |
Assessment-day checklist
- Lay the paperwork out: guarantees, invoices, MCS and Benchmark certificates, building control sign-offs, dated photos.
- Clear access to the loft hatch, the boiler, the hot water cylinder and the meter.
- Leave conservatory doors closed and in place — a thermally separated conservatory stays out of the calculation.
- Point out anything non-obvious: second heating zones, underfloor insulation, the extension's build year, that the flat roof was insulated during re-roofing.
- Confirm every lamp is low-energy — it is the one item you can still fix five minutes before the doorbell rings.
Choosing the assessor and timing the booking
Domestic EPCs cost £60–£120 and any accredited domestic energy assessor can lodge one — the register of accredited assessors is searchable on gov.uk. Book after all measures are complete rather than re-certifying per measure. If your project followed the cheapest-first ranking and the cost ladder, you should arrive with two or three points of headroom over 69 — sensible margin, because recorded inputs vary slightly between assessors and published point gains are typical rather than guaranteed.
One more planning note: if your current certificate pre-dates 2025, the RdSAP 10 changes described in the SAP points guide may have moved your true starting score in either direction. Borderline homes — 65 and above — sometimes cross into C on reassessment alone, which is why we suggest borderline owners and landlords consider a reassessment before commissioning any work.