What Happens During an EPC Assessment: A Step-by-Step Guide

During an EPC assessment, an accredited assessor visits your property and records the walls, roof, floor, glazing, heating, hot water and lighting. The visit takes 45 minutes for a studio flat and up to 90 minutes 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, acceptable evidence can help the assessor record installed insulation rather than rely on the applicable default.

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.

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. The applicable controls are recorded; their effect depends on the heating system and the whole-property calculation.

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 lamps in permanent fittings. Under the current RdSAP 10 conventions, LED and fluorescent/CFL lamps are recorded separately; halogen and tungsten lamps are recorded as incandescent. There is no fixed point gain for changing the lamps in every property.

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?

Clear loft access and paperwork laid out in advance keep the visit at the shorter end.

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. Differences in evidence and recorded data can change the result, but there is no fixed point change applicable to every property. 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 is recorded using the applicable evidence and methodology rules. Suitable documentary evidence may support an insulation entry; otherwise defaults may apply. Provide safe access where possible, without assuming a fixed effect on the score.

"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 official GOV.UK EPC register, and we send you the link to it once it is live. 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 guide prices by internal floor area, or get your exact quote directly.