What an Assessor Does

An assessor's core job is to observe, record and calculate. They visit the home, measure its dimensions, identify its construction and systems, and enter those observations into approved software under a prescribed method, producing a standardised rating and recommendations. That job is observational and calculative: the assessor describes what is there, applies the method's rules, including defaults where observation is impossible, and lodges the result. They do not design retrofit work, install measures, value the property, or survey its structural condition, and those boundaries matter when reading a certificate.

Measurement is the foundation. The assessor measures floor areas, room dimensions and glazing areas, notes storeys and extensions, and records how spaces are heated or unheated. Errors here propagate through the whole calculation, because area scales many results, so a careful assessor measures rather than copying an old plan or an estate agent's floor area. Where a measurement is uncertain, for example a room with sloping ceilings or an irregular extension, the assessor should note the basis used.

Construction and system identification follow. The assessor records wall, roof and floor types, insulation where it can be observed or evidenced, window types and draught sealing, heating and hot water systems, controls, lighting and ventilation. Where a feature cannot be seen, such as insulation inside a closed cavity or under a floor, the assessor follows the method: using documentary evidence where available, such as invoices or building records, and otherwise applying the method's default for the age and type. Defaults are a legitimate part of the method, but they should be recognisable as defaults in the input record, not presented as observed facts.

Calculation and lodgement complete the job. The software applies standard occupancy, heating patterns and fuel prices to the recorded inputs, producing the rating, the figures and the recommendations, and the assessor lodges the certificate on the relevant register. The assessor should check the result for plausibility before lodgement, for example whether a rating looks inconsistent with the building type, and should be able to explain the main drivers of the result to the client.

Before the visit, prepare access and information: clear access to loft hatches, boiler, meters and all rooms, plus any documents about past work, such as insulation invoices, window specifications or heating system manuals. During the visit, expect the assessor to measure, photograph and note features, and to ask about construction history where documents are missing. After the visit, expect a lodged certificate and, where the scheme provides it, access to input details, within the scheme's normal timescale.

Also be clear about what the assessor will not do. They will not open up walls or floors, test systems beyond visual identification, advise on the best retrofit package in design detail, or guarantee bills or comfort. Where those services are needed, a retrofit assessor, designer, installer or building surveyor, as appropriate, should be engaged separately, and the EPC assessment can serve as a starting input for their work.

By place, assessor practice follows the national scheme. UK domestic assessors work to SAP-based methods and accreditation schemes, with site visits as the norm for EPCs. EU certifiers work to national methods under the EPBD, with varying requirements for site visits, data collection and lodgement. Remote or desktop assessments, where a scheme permits them for particular purposes, are a separate category with their own limits, described in a companion guide, and they should not be confused with a full site assessment.

Within the UK, domestic and non-domestic assessors follow different routes and methods, and an assessor for a home should not be assumed qualified for a commercial building. Special cases, such as new builds, historic buildings or complex multi-unit blocks, may also require particular experience or strands, and clients should check that the assessor's experience matches the building.

A common mistake is to brief an assessor vaguely and then be disappointed that the certificate does not answer a retrofit or condition question. The brief determines the output: a transaction EPC answers a standardised rating question, while a retrofit assessment answers a design question, and the two need different scopes. Another mistake is to withhold information about past work, which forces the assessor to default where documentary evidence would have improved accuracy.

A further mistake is to pressure an assessor for a better rating. An assessor who changes inputs to flatter a result breaks the method and the accreditation rules, and the resulting certificate is unreliable and vulnerable to correction. A good assessor explains inputs and defaults honestly, even where the rating disappoints.

Keep the assessor's details, the lodged certificate, any input schedule and the documents you provided together. If a later professional queries an input, that bundle shows what the assessor saw and what evidence was available at the time.

Questions to ask include: what will you measure and record, what defaults might apply to this home, and what will I receive and when. Those questions set clear expectations for any assessment.