The Ultimate Guide to an EPC: What Happens on a Domestic Energy Assessment

Why the Site Visit Is the Whole Job

Watch an experienced Domestic Energy Assessor work through a Victorian terrace and one thing becomes obvious: the certificate is only as good as the hour spent on site. Everything the software eventually calculates — heat loss, hot water demand, lighting energy, the SAP rating band — traces back to what was observed, measured, and photographed at the property.

That matters for anyone revising for the DEA qualification, and it matters for Retrofit Assessors too, because a PAS 2035 Retrofit Assessment begins with exactly this RdSAP survey before the condition report, occupancy assessment and ventilation checks are layered on top.

Tip: When you revise, do not treat RdSAP conventions as abstract rules. Attach every convention to the physical thing you would be standing in front of — a wall, a boiler badge, a cylinder thermostat.

Arriving: ID, Consent and Explaining the Photographs

The visit opens with identification. A DEA carries an ID card showing their name and accrediting body — Stroma in the video’s case — with a contact number so the householder can verify the assessor is genuine.

The assessor then explains what is about to happen, and specifically warns that photographs will be taken throughout. This is not optional courtesy. Photographic evidence exists so the accreditation scheme’s auditors can check the assessment was carried out properly, and every scheme operates a quality assurance regime built around it.

• Windows and doors
• Light fittings
• Heating appliance data badges
• Gas and electricity meters
• Any inaccessible element, photographed as inaccessible

Stage One: Construction and Insulation

The first survey task is establishing wall construction. On the Victorian terrace in the video, the assessor is looking for solid brickwork, and checking whether external wall insulation has been retrofitted.

Two simple techniques do most of the work:

1. Wall thickness measurement at a reveal — an unusually wide measurement suggests an added insulation layer.
2. The tap test — knocking on the wall to hear whether it sounds hollow.

For cavity construction, the assessor looks for drill patterns in the mortar joints alongside the thickness check, since retrofitted cavity fill leaves visible evidence.

Where insulation is present but its thickness cannot be established by inspection, the assessor asks the householder for installation certification. That documentation gives the actual insulation specification and U-value, which produces a far more accurate certificate than the age-band default RdSAP would otherwise apply.

Did you know? The default assumption is almost always worse than a well-evidenced reality. Encouraging householders to dig out paperwork is one of the few things a DEA can do that genuinely improves a rating without a single new measure being installed.

If you want the fabric theory behind these inspections, our building physics revision guide covers U-values, thermal bridging and heat-loss fundamentals.

Stage Two: Roofs and Loft Spaces

Loft access is one of the biggest single influences on the rating, which is why the assessor asks for the hatch to be clear and reachable. Measuring insulation depth directly is always preferable.

The property in the video has a loft conversion instead of a conventional loft, so the assessor inspects the eaves cupboards at the sides of the roof room, looking for visible insulation. Here the space had been sealed off entirely. The correct response was:

• Photograph the sealed void to evidence that inspection was not possible
• Rely on the documentary evidence available for the recent construction
• Complete the data entry back at the office

That sequence — inspect, evidence, record the limitation honestly — is exactly what auditors want to see, and it is the same discipline you need on any element you cannot physically reach.

Stage Three: Heating, Controls and Hot Water

Space heating dominates the calculation, so this stage takes the most care. The assessor records:

Element	What is recorded	Why it matters
Fuel and generator	Gas, electric or renewable	Sets fuel cost and carbon factors
Emitter type	Radiators, underfloor, warm air	Affects system efficiency
Boiler identity	Make and model from the badge	Matches the appliance to its efficiency database entry
Controls	Room stat, TRVs, programmer, zoning	Directly adjusts the control category
Cylinder	Size, insulation type and thickness	Drives hot water losses
Cylinder stat	Present or absent	Affects hot water control

The boiler check is deliberately minimal — open the front cover, read the model number, close it. No panels come off, no tools are used. In the video the appliance is an Ecotec Plus 630, and that identifier is all the software needs to look up the correct efficiency.

Controls are where properties gain or lose points quietly. The video’s property has thermostatic radiator valves upstairs, an underfloor manifold with per-room control downstairs, and full zonal programmable thermostats in every room — a combination that genuinely lifts the rating because time and temperature can be controlled independently in each space.

For hot water, the assessor records cylinder size, whether insulation is factory-applied or a jacket, its thickness where determinable, and the presence of a cylinder thermostat. Insulation detail is another case where manufacturer documentation beats guesswork.

If heating technology is your weaker area, the write-up on heat pumps and air-to-air systems in UK retrofit covers how these systems are treated in assessment and retrofit design.

Stage Four: Glazing, Bathrooms and Lighting

Glazing is recorded by type — single, double or triple — along with frame material and, for the multiple glazing types RdSAP distinguishes, the glazing gap. The wooden sash windows in the video are entered as wooden double glazing, with a photograph taken to evidence both the type and the gap.

Bathrooms need only a count of baths and showers, photographed as evidence, since these feed the hot water demand calculation.

Lighting has a small but real effect. The assessor counts fixed light fittings and identifies whether each is low energy — LED or compact fluorescent — or an old-style halogen or incandescent. Where a property has a mix, the count is broken down room by room and totalled in the software.

Remember: Small-impact items still get audited. A quick count done carelessly is exactly the kind of error a QA sample picks up.

Stage Five: Meters and Renewables

The gas and electricity meters are checked and photographed. For electricity, the assessor notes whether it is a single-rate or dual-rate (Economy 7 style) meter, since that changes the tariff assumptions. The gas meter record simply confirms mains gas is present. Both photographs also serve as proof of the visit for audit purposes.

Renewables were absent in this London property, but where they exist the approach is the same as for insulation: ask for documentation. Nobody is climbing onto a roof to measure panel dimensions in a half-hour survey. An MCS certificate provides the system size and specification the software needs, and that is the evidence auditors expect to see.

Stage Six: The Floor Plan

The floor plan is described as the most time-consuming part of the visit, at roughly 10 to 15 minutes for an average property. It is not an estate agent’s marketing plan. The assessor is producing a measured sketch that yields:

• Total floor area for each storey
• Areas divided by construction type where they differ
• Identification of heat loss walls — the walls exposed to the external environment

A laser measure is used against one wall to read across to the opposite face, and the dimensions are recorded on the sketch as the assessor works around each floor.

Getting heat loss perimeter and party wall identification right is one of the most common sources of assessment error, and it is worth drilling deliberately. Our guide to the DEA exam workflow breaks down how to sequence survey tasks so nothing gets missed.

After the Visit

The whole site process takes about an hour for an average property. The assessor returns to the office, enters everything into RdSAP software, and the certificate is typically lodged and returned within a few days.

The sequence to memorise is straightforward:

1. Identify and explain
2. Construction and insulation
3. Roof and loft
4. Heating, controls and hot water
5. Glazing, bathrooms and lighting
6. Meters and renewables
7. Floor plan and heat loss walls
8. Data entry and lodgement

How RetrofitReady Fits Into This

Everything in this survey maps directly onto the DEA syllabus — safe working practice on arrival, preparation and consent, the assessment itself, and EPC production back at the office.

RetrofitReady: DEA carries 360 DEA questions covering safe practice, preparation, assessment, EPC production and focused supplement units — the exact stages the video walks through. Practise the survey sequence by topic first, then move to mixed sets once each stage is secure.

If your route continues into PAS 2035 work, the same survey underpins a Retrofit Assessment. Retrofit Assessor adds 480 questions across eight syllabus areas, including multi-answer and diagram-based practice, and RetrofitReady: Coordinator offers 600 PAS 2035 questions across domestic retrofit, risk, fabric, services, ventilation, MTIP and monitoring. For those working on the installation side, RetrofitReady: Installer provides 480 PAS 2030 questions across the two core units and six fabric measures.

Across the suite that is 1900+ questions across 4 apps, structured so you can move between qualification routes without relearning the platform.

Tip: Multi-answer questions catch people out on survey-procedure topics, because several steps are genuinely correct. Our guide to approaching multi-answer retrofit questions explains how to work through them.

Start with the free quizzes, browse the full RetrofitReady app suite, or check the recommended books and tools page for survey equipment and reference texts worth owning.