The EPC Assessment Process — What Domestic Energy Assessors Check and How to Prepare

What Is a Domestic Energy Assessor?

An EPC can only be produced by an accredited Domestic Energy Assessor (DEA). DEAs are qualified professionals accredited by one of several government-approved schemes (such as Elmhurst Energy, STROMA, or Quidos). They must carry professional indemnity insurance and follow a standardised methodology — SAP (Standard Assessment Procedure) — to calculate your property's energy performance.

EPC assessments typically take 30–60 minutes for a standard residential property. The assessor does not test or measure actual energy consumption — the rating is based on the physical characteristics of the building, not on utility bills.

What the Assessor Records

1. Property Type and Age

The construction era (pre-1900, 1900–1929, 1930–1949, 1950–1966, 1967–1975, 1976–1982, 1983–1990, 1991–1995, 1996–2002, post-2002) determines the default assumptions SAP applies to elements the assessor cannot directly observe. Older construction eras have lower assumed insulation levels.

2. Dimensions

The assessor will measure the floor area of each habitable room, plus the wall, window, and roof areas. These are used to calculate the building's total heat loss surface. Accuracy here matters — an error in floor area directly affects the EPC score.

3. Wall Construction

The assessor will identify whether walls are cavity or solid, the main material (brick, stone, timber frame, concrete), and whether any insulation is present. Evidence of insulation — a cavity fill guarantee certificate, building regulations completion certificate, or an installer's invoice — should be shown to the assessor. Without evidence, insulation may be scored as absent or unknown.

4. Roof and Floor Insulation

Loft insulation depth is measured or estimated. If the loft is accessible, the assessor will look. If not, they may rely on your evidence. Again: certificates and invoices matter. Ground floor construction (solid concrete vs suspended timber) affects how floor insulation is assessed.

5. Windows and Doors

The assessor records glazing type (single, double, triple), approximate installation date, and frame material for each opening. Glazing installed before 2002 is assumed to be less thermally efficient than post-2002 units under SAP assumptions.

6. Heating System

This is one of the most significant inputs. The assessor records:

• Boiler make, model, and age (to look up efficiency in the PCDB)
• Fuel type (mains gas, LPG, oil, electric, etc.)
• Heating controls (programmer, room thermostat, TRVs)
• Hot water system (combi, system with cylinder, immersion)
• Any secondary heating (open fire, gas fire, electric heaters)

7. Renewable Energy Systems

Solar PV panels, solar thermal collectors, and wind turbines are recorded. For solar PV, the assessor notes the system size (kWp), orientation, and tilt angle.

8. Lighting

The proportion of fixed low-energy lighting (LED or CFL fittings) throughout the property is noted. Having all LED lighting throughout adds a small number of SAP points.

Common Reasons for an Underscored EPC

The most frequent causes of a lower-than-deserved EPC are:

• **Insulation improvements not evidenced**: If you have had cavity wall insulation or loft insulation installed but cannot show the guarantee certificate, the assessor may record it as unfilled/uninsulated. Locate or request duplicate certificates from your installer.
• **Old boiler model not in PCDB**: Older boilers default to conservative efficiency assumptions. If your boiler was replaced but there is no visible signage or documentation, the assessor may assume the old appliance.
• **Conservatories incorrectly included or excluded**: An unheated conservatory separated by a door is typically excluded from floor area calculations; one that is heated and open to the house is included. Misclassification affects the result.
• **Solar panels not registered**: If panels were installed after the last EPC, the current certificate will not reflect them. Commission a new EPC.

How to Prepare for Your EPC Assessment

1. Locate installation certificates for all insulation and heating improvements

2. Ensure the assessor has access to all rooms, the loft, and the boiler

3. Note the make and model of your boiler and any thermostats or controls

4. Check that LED bulbs are fitted in all accessible light fittings

5. Have invoices or building control completion certificates for any extensions ready

A well-prepared property assessment is more likely to accurately reflect all the improvements you have made. If you believe your EPC is incorrect, you can raise a complaint with the accreditation scheme that certified the assessor.

Use the [EPC Improvement Calculator](/epc-improvement-calculator) to estimate your current SAP score and model the impact of improvements before commissioning a new assessment.