What happens in a non-domestic EPC assessment

Where defaults come from, and why they hurt

When the assessor cannot establish something, the methodology does not let them guess optimistically. It requires a default, and defaults are deliberately cautious, because the alternative would be a system that rewards vagueness.

So a building with excellent insulation that nobody can evidence is modelled as though it has the minimum. A boiler with a good efficiency figure that nobody can produce documentation for is modelled at a conservative assumption. The building is better than its certificate, and the gap is pure paperwork.

This is the single most useful thing on this page: finding the documentation is free, and it is sometimes worth a band.

What to have ready

Construction drawings and specifications, particularly anything showing insulation types and thicknesses.
Building control or completion documentation from any extension, refurbishment or fit-out.
Manufacturer data and commissioning certificates for boilers, chillers, air handling units and controls.
Lighting schedules: fitting types, wattages and the control strategy, by zone.
Any previous EPC and its recommendation report, which tells the assessor what was there before.
Evidence of anything installed since the last assessment, which is frequently the thing nobody mentions.

What the assessor is doing on site

Measuring and zoning the building, identifying the construction of each element, recording the fixed services and their controls, and photographing evidence. They are not testing performance, and they are not there to advise you on what to fix.

Access matters. Plant rooms, roof voids, risers and locked areas that cannot be inspected become defaults, with the consequences described above. An assessment where half the building was inaccessible produces a worse rating than the building deserves.

The recommendation report

An assessment produces a recommendation report alongside the certificate, listing improvements with indicative payback categories. It is a useful starting point and it is a generic one: it comes out of the methodology rather than out of someone thinking about your building commercially.

Treat it as a prompt, not a plan. A contractor pricing the actual work, and an assessor modelling what each measure would do to your specific building, will tell you considerably more.

Assess before you spend

If you are about to commit meaningful capital, having the building modelled first tells you which measures move your rating rather than which measures move a typical rating. That is a modest fee against a large spend, and it occasionally changes the plan completely.

Questions we get asked

Can an EPC rating be improved without changing the building?

Sometimes, yes, and legitimately. Where evidence is missing the methodology requires cautious defaults, so a building with good insulation or efficient plant that nobody can document is modelled as though it does not have them. Producing the paperwork can be worth a band and costs nothing.

What should I have ready for the assessor?

Construction drawings and insulation specifications, building control documentation from any works, manufacturer and commissioning data for plant, lighting schedules with wattages and controls by zone, and any previous EPC. Also access to plant rooms, roof voids and risers.

Does the assessor test how the building performs?

No. They collect data about the building and its fixed services and enter it into approved modelling software. It is a modelled asset rating, not a measurement of how the building actually performs in use.

Should I get an assessment before doing the work?

If you are spending meaningful money, yes. A pre-works assessment tells you which measures move your building rather than a typical building, which is a small fee against a large capital decision.

Last updated 8 October 2026.