During an EPC assessment, an accredited assessor visits your property and records around 40 data points covering walls, roof, floor, glazing, heating, hot water, and lighting. The visit takes 30 minutes for a studio flat and up to 2 hours for a large detached house. The rating is generated by RdSAP 10 software after the visit, not estimated on-site.
What the assessor looks at, step by step
1. External inspection and dimensions
The assessor starts by recording:
- Floor area, measured room by room across all heated spaces. Unheated garages and outbuildings are excluded.
- Construction age band, pre-1900, 1900–1929, 1930–1949, and so on. Age band determines the assumed U-values for walls and floors when direct measurement isn't possible.
- Property type, detached, semi-detached, end-terrace, mid-terrace, or flat (including floor position). A ground-floor flat has more exposed surfaces than a mid-floor flat and typically scores slightly lower for fabric heat loss.
2. Walls
Wall construction is one of the most heavily weighted inputs in the SAP calculation:
- Solid brick (pre-1919): Typical 9-inch single-leaf construction. The assessor measures wall thickness at a window reveal, under 260mm confirms solid.
- Cavity brick-and-block (post-1920): Typically 280–330mm at the reveal. The assessor records whether cavity insulation is present, confirmed by a BBA or CIGA certificate, visible drill holes (mortared over post-installation), or occasionally a drill test.
- External wall insulation: Visually identifiable from rendered insulation on the outside. Depth and product type recorded where accessible.
If insulation cannot be confirmed, the software defaults to uninsulated, a conservative assumption that lowers the score. Providing paperwork overcomes this default.
3. Roof and loft
- Loft insulation depth, measured directly with a ruler or probe. Standard reference depths: 0, 25, 50, 75, 100, 150, 200, 270mm+.
- Roof type, pitched, flat, or room-in-roof (where habitable space extends into the roof zone).
- Accessibility, if the loft is inaccessible, the software defaults to a regional age-band assumption, typically zero or minimal insulation for pre-1980 properties.
Tip: Clear the loft hatch before the assessment. A physically accessible loft with a visible measured depth gives the assessor confirmed data rather than a conservative default.
4. Ground floor
- Floor construction, suspended timber, solid concrete, or insulated slab.
- Insulation, recorded only if confirmed by building records or direct observation.
Assessors cannot lift carpet or flooring. If your property has insulated suspended timber floors (e.g. with rockwool between joists), provide the builder's records, the default assumption is uninsulated, which can cost 1–3 SAP points.
5. Windows and doors
- Glazing type, single, double, or triple.
- Installation date, affects the assumed U-value. Post-2002 double glazing is credited as modern. Pre-2002 double glazing uses an older, higher heat-loss U-value.
- Frame material, uPVC, timber, or aluminium.
FENSA certificates confirm installation date. If you have newer double glazing but the paperwork is missing, the assessor uses a more conservative U-value, FENSA confirmation is worth keeping.
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6. Heating system
The most heavily weighted factor in the SAP calculation:
- Main heating fuel, gas (grid), electricity, oil, LPG, biomass, heat pump, district heating
- System type, combi boiler, system boiler with cylinder, storage heaters, heat pump
- Boiler efficiency, modern condensing boilers (post-2005) achieve 89–92%. Pre-2005 non-condensing: 70–80%. The assessor records the make and model from the boiler casing.
- Heating controls, room thermostat, programmer, thermostatic radiator valves (TRVs), weather compensation. Each type is recorded separately and adds points.
7. Hot water
- Generation method, combi boiler (instant), cylinder with immersion, solar thermal, heat pump
- Cylinder, size, jacket insulation depth
- Immersion heater, single or dual element
Electric immersion heaters as the primary hot water source score very poorly. A combi boiler with no cylinder scores better.
8. Renewable energy
- Solar PV, rated output (kWp), roof pitch, orientation (azimuth), shading assessment
- Solar thermal, collector area, orientation
- Battery storage, not currently credited under RdSAP 10
A south-facing 3.5kWp system with no shading at 35° pitch gets full credit. East- or west-facing, or heavily shaded systems, receive reduced credit proportional to their actual generation estimate.
9. Lighting
The assessor counts all fixed light fittings and records how many are low-energy (LED or CFL). Under RdSAP 10 (October 2025), all low-energy fittings are assumed to be LED, the distinction between CFL and LED was removed from the standard. A property where all fittings are LED gets maximum lighting credit: worth 2–4 SAP points for a typical 3-bed house.
Tip: Ensure all fixed fittings, ceiling roses, downlights, bathroom lights, external porch lights, have LED bulbs installed before the assessment. Plug-in table lamps are not counted.
How long does the assessment take?
| Property type | Typical duration |
|---|---|
| Studio or 1-bed flat | 30–45 minutes |
| 2-bed flat or house | 45–60 minutes |
| 3-bed house | 60–90 minutes |
| 4-bed house | 75–90 minutes |
| 5+ bed house | 90–120 minutes |
Add 15–20 minutes for difficult loft access, missing documentation, or multiple outbuildings.
What to have ready
You do not need to clean or decorate. The assessor measures physical characteristics. What genuinely helps:
- All rooms accessible, including room-in-roof spaces, utility rooms, and any outbuildings within the heated zone
- Clear loft hatch, the assessor needs to enter or at least lean in to measure insulation depth
- Boiler accessible, make and model on the casing are noted; the installation date may be on the commissioning paperwork inside the casing
- Any building certificates:
- FENSA certificate (windows), overrides conservative glazing U-value default
- CIGA or BBA certificate (cavity wall insulation), confirms fill, overrides uninsulated default
- Building control sign-off (floor insulation), confirms depth, overrides uninsulated default
- Meter type, smart, Economy 7 (two-rate), or standard single-rate
Common surprises and what to know in advance
"I was told my property would get a C but it came back D." The rating is generated by software, not by the assessor's estimate. Small differences in recorded data, boiler model, insulation depth measured vs. estimated, glazing date, can move the score by 3–5 points. The assessor's pre-visit estimate is informal; the software result is the official rating.
"The assessor couldn't get into my loft." An inaccessible loft defaults to zero or minimal insulation. Even a blocked hatch can cost 4–6 SAP points. Clear it before the visit.
"I have insulation but the assessor didn't credit it." Without physical confirmation or paperwork, the assessor cannot override the default. Provide CIGA, BBA, or building control certificates for any insulation improvement that was professionally installed.
"My neighbour got a higher rating for an identical house." Small data differences, boiler replacement date, insulation depth measured on a particularly sunny or compacted area, genuinely move the score. If you believe an error was made, ask the assessor to recheck specific fields before the certificate is lodged.
After the assessment
The assessor enters data into Elmhurst's RdSAP 10 software, which calculates the SAP score and generates the certificate. The certificate is lodged on the government register and emailed to you. It is valid for ten years, see our EPC validity guide for when renewal is required.
We serve all London boroughs including Hackney, Tower Hamlets, Ealing, Bromley, and 28 others. See our pricing page for fixed costs by property size, or book an assessment directly.
Frequently asked questions
- Someone must provide access to all rooms including the loft, boiler cupboard, and any outbuildings within the heated envelope. This can be a managing agent, tenant, or keyholder, you don't have to be there personally.
- No. Assessors work non-invasively and will not move furniture, lift flooring, or open boiler casings. If floor insulation cannot be confirmed because the floor is carpeted and inaccessible, the software applies a default (uninsulated) assumption, which can lower the rating. Provide any building records that confirm floor insulation.
- Improvements already in place, LED bulbs, new loft insulation, a new boiler, will be recorded and credited. There is no benefit to making changes specifically for the assessor that you would then reverse. Focus on permanent improvements, ideally done before commissioning the assessment.
- With L&D Energy, the certificate is issued within 72 hours of the assessment and simultaneously lodged on the government EPC register. Next-day certificates are available for an additional £12.
- Ask the assessor to re-check specific data points you believe were recorded incorrectly. If you believe there was an error, you can raise a complaint through Elmhurst Energy (our accreditation scheme). You cannot change the rating simply because you dislike it, the software applies fixed RdSAP 10 methodology.
Related guides
EPC Ratings Explained: A to G Bands, SAP Scores and What Each Means
EPC ratings run from A (most efficient) to G (least). This guide explains the A–G band scale, SAP scores, where most London homes sit, and what your rating means for selling or renting.
How Long Does an EPC Last? Validity, Renewal and Mortgage Implications
An EPC is valid for 10 years. Here is when you can use an old certificate, when you must get a new one, how to check expiry, and why some mortgage lenders now require a current EPC.
RdSAP 10: What Changed in October 2025 and What It Means for Your EPC
RdSAP 10 replaced RdSAP 9.94 in October 2025, the first major methodology update since 2012. Here's what changed, which properties are affected, and why some ratings have shifted.
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