If you’ve never had an EPC assessment before, the process can feel mysterious. An assessor arrives, walks around your property with a measuring device and camera, asks some questions, and leaves 30–60 minutes later.

But what are they actually doing? Why are they measuring certain walls and not others? What are the photographs for? And how does all this data translate into your final A-to-G rating?

This post takes you step-by-step through a domestic EPC assessment, explaining what assessors are looking for, how the measurement process works, and why seemingly small details can significantly affect your rating.

Key Takeaways

• Assessments typically take 30–60 minutes depending on property size.
• Assessors measure floor area, wall dimensions, window sizes, and ceiling heights.
• Photographs provide evidence for auditing and verify construction details.
• The process is non-invasive — no drilling, lifting floors, or opening sealed spaces.
• Data collected feeds into RdSAP software, which calculates your rating based on modelled energy performance.

Before the Assessment: What You Need to Prepare

A successful EPC assessment starts with preparation. While you don’t need to tidy or redecorate, there are a few things that help the process run smoothly.

• Provide access to all rooms: The assessor needs to measure every habitable space, including bedrooms, living areas, kitchens, and bathrooms.
• Ensure loft access is available: If your property has a loft hatch, the assessor will need to inspect insulation levels.
• Locate boiler information: If possible, find the boiler model number or any installation certificates.
• Gather documentation: Invoices, certificates, or photographs of energy efficiency improvements (insulation, glazing, heating upgrades) help ensure they’re recognised in the assessment.

Step 1: External Inspection and Property Orientation

The assessment begins outside. The assessor will photograph the front and rear elevations of the property and note the orientation (which direction the property faces).

Why this matters: Orientation affects solar gain. South-facing properties receive more sunlight, which the EPC software models as a positive factor for heating. The software also needs to understand the property’s exposure to wind and weather.

The assessor will also identify the construction type — solid brick, cavity wall, rendered, stone-built, or timber-framed. This is typically evident from external appearance, though it may require confirmation during the internal inspection.

Step 2: Measuring Internal Floor Areas

Once inside, the assessor measures the floor area of every room. This is done using a laser measuring device for accuracy and speed.

The assessor measures length and width to calculate square metreage for each room, including hallways, landings, and storage areas. Measurements are taken to walls, not skirting boards, to ensure consistency.

Why this matters: Total floor area is a critical input into the EPC software. Larger properties require more energy to heat, so floor area directly affects the modelled energy consumption and, consequently, the rating.

The assessor also notes ceiling heights in each room. Standard ceiling height is assumed to be around 2.4 metres, but Victorian properties, conversions, and extensions often have higher or lower ceilings, which affects heating demand.

Step 3: Recording Wall Construction and Insulation

The assessor identifies the construction type of external walls — solid brick, cavity wall (filled or unfilled), stone, timber frame, or other materials.

In Bristol, many Victorian and Edwardian terraces have solid brick walls (9 inches thick, no cavity). Post-1930s properties typically have cavity walls, which may or may not be insulated.

How this is determined: Wall thickness can sometimes be measured at door or window reveals. Brick bonding patterns visible on external walls can indicate solid or cavity construction. However, if the construction type is unclear and no documentation exists, the assessor must make a conservative assumption based on the property’s age and visible evidence.

Why this matters: Solid walls have poor thermal performance unless externally or internally insulated. Cavity walls perform better, but unfilled cavities still lose significant heat. The software applies different U-values (thermal transmittance ratings) based on wall construction, which heavily influences the overall rating.

Step 4: Windows and Glazing

The assessor measures the size of every window and records the glazing type — single-glazed, double-glazed, or triple-glazed. Frame material (timber, uPVC, aluminium) is also noted.

With the introduction of RdSAP 10 software, windows are now modelled more accurately than in previous versions. The software calculates the percentage of glazing relative to floor area and applies thermal performance values based on glazing type and frame material.

Why this matters: Windows are a major source of heat loss. Single-glazed windows significantly reduce EPC ratings, while modern double or triple glazing improves performance. The assessor will photograph windows to provide evidence of glazing type for auditing purposes.

Step 5: Heating System and Controls

The assessor inspects the main heating system — typically a gas boiler in Bristol properties, though electric heating, oil boilers, or heat pumps may also be present.

The assessor records the boiler make and model (usually found on a data plate on the boiler itself), the fuel type (mains gas, LPG, oil, electric), and the presence of heating controls including programmer or timer, room thermostat, and thermostatic radiator valves (TRVs).

Photographs are taken of the boiler and control devices to verify the information entered into the software.

Why this matters: Boiler efficiency is one of the most significant factors in EPC ratings. Older non-condensing boilers (pre-2005) perform poorly in the software model, while modern condensing boilers achieve much higher efficiency ratings. Heating controls improve modelled performance by demonstrating that the system can be managed effectively to reduce waste.

Step 6: Hot Water System

The assessor identifies how hot water is provided — typically from the main boiler in a combi or system boiler setup, or from an electric immersion heater in properties without gas central heating.

If the property has a hot water cylinder, the assessor notes whether it has insulation (a jacket or factory-fitted foam insulation) and whether the cylinder thermostat is present.

Why this matters: Hot water production is energy-intensive. Efficient systems (combi boilers, well-insulated cylinders) perform better in the software model than uninsulated cylinders or electric-only systems.

Step 7: Lighting

The assessor counts the number of fixed light fittings in the property and records how many use low-energy bulbs (LEDs or compact fluorescents) versus standard bulbs (halogen, incandescent).

Only fixed fittings are counted — table lamps and portable lights are excluded.

Why this matters: The software calculates the percentage of low-energy lighting. Properties with a high percentage of LED or low-energy bulbs receive a ratings boost. This is one of the easiest and cheapest ways to improve an EPC score.

Step 8: Loft Inspection

If the property has an accessible loft hatch, the assessor will open it and inspect the insulation depth. A measuring device or ruler is used to check the thickness of insulation material.

The assessor photographs the loft insulation to provide evidence for auditing.

Why this matters: Loft insulation is highly effective at reducing heat loss. The software applies different thermal performance values based on insulation depth. Around 270mm is considered optimal in current standards. Properties with little or no loft insulation score poorly, while well-insulated lofts improve the rating.

If the loft is not accessible (sealed hatch, no hatch, boarded loft with insulation hidden), the assessor cannot verify insulation depth and must rely on documentation or make a conservative assumption.

Step 9: Floor Insulation (If Accessible)

Ground floor insulation is difficult to verify in most properties unless there is visible evidence (e.g., insulation visible in an accessible underfloor void, or documentation from a recent installation).

The assessor notes the floor type — solid concrete, suspended timber, or screed — and records any visible or documented insulation.

Why this matters: Uninsulated suspended timber floors (common in Victorian properties) allow significant heat loss. If insulation has been added but cannot be verified, it won’t be credited in the assessment.

Step 10: Photography for Audit Evidence

Throughout the assessment, the assessor takes photographs of key features. These images are not used in the final certificate provided to you — they are stored as evidence in case the assessment is audited by the accreditation scheme.

Typical photographs include front and rear elevations, boiler and controls, windows and glazing types, loft insulation, hot water cylinder, and any unusual or non-standard features.

Why this matters: Accreditation schemes (Elmhurst, Quidos, Sterling) randomly audit assessments to ensure accuracy and compliance. If an assessor claims a property has double glazing or 270mm of loft insulation, they must have photographic evidence to support the claim. Without this evidence, the assessor could face disciplinary action.

After the Visit: How the Data Becomes a Rating

Once the site visit is complete, the assessor inputs all collected data into RdSAP software (Reduced Data Standard Assessment Procedure). This is the government-approved calculation engine used for all domestic EPCs.

The software models the property’s energy performance based on its size, construction, heating system, insulation levels, glazing, and orientation. It calculates annual energy costs and CO2 emissions, then assigns a rating from A (most efficient) to G (least efficient).

The certificate also includes a list of recommended improvements ranked by cost-effectiveness, showing how much each upgrade could improve the rating.

The final EPC is lodged with the government’s central register and typically delivered to you within 48 hours of the assessment.

What the Assessor Is NOT Doing

It’s worth clarifying what an EPC assessment is not:

• Not a structural survey: The assessor is not checking for damp, structural defects, or building faults.
• Not a valuation: The assessment does not determine the property’s market value.
• Not an energy bill audit: The rating is based on modelled performance, not your actual energy consumption or bills.
• Not invasive: The assessor will not drill walls, lift floors, or open sealed spaces.

Common Questions About the Assessment Process

Do I need to be present during the assessment?

Someone must provide access to the property, but you don’t need to remain for the full visit. Many homeowners provide access and then leave the assessor to work independently.

Will the assessor move furniture or belongings?

No. Assessors measure around furniture and do not move personal belongings. If a room is heavily cluttered, the assessor may need to estimate dimensions, which could affect accuracy.

Can I ask the assessor questions during the visit?

Yes. Most assessors are happy to explain what they’re doing and answer questions about the process or potential improvements. However, the final rating cannot be guaranteed until the data is entered into the software.

What if I disagree with the rating?

If you believe the assessment is inaccurate — for example, if improvements were missed or incorrect data was recorded — you can contact the assessor to discuss. If the issue cannot be resolved, you can lodge a complaint with the accreditation scheme or commission a new assessment.

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Now that you understand the process, book your EPC assessment with confidence. We’ll ensure your property is assessed accurately and professionally, with every improvement properly recognised.