The short answer
For a new dwelling in England, energy performance is checked twice: before work starts using the design specification, and again when work is complete using the dwelling as actually constructed. The same approved calculation methodology must be used at both stages.
The design-stage calculation tests the proposed dwelling against its target primary energy rate, target emission rate and target fabric energy efficiency rate. The design-stage BREL report can be used to give building control those results together with the specification used in the calculation.
What changes at the as-built stage
The completion calculation must reflect the finished building. It incorporates construction changes and the measured air permeability, rather than simply reissuing the original design model. Building control can accept the as-built BREL report from an accredited energy assessor, accompanied by the required photographic evidence.
Changes to insulation, windows, thermal junctions, heating, hot water, ventilation, lighting, renewables or airtightness can alter the result. A dwelling that passed at design stage is not automatically compliant as built if products or details changed on site.
How to avoid a late compliance problem
Tell the SAP assessor about substitutions before they are installed. Keep revised drawings, product data, invoices, commissioning information, air-test results and clear photographs of relevant details while they remain visible.
The useful workflow is design assessment, controlled change management, evidence capture during construction and a final as-built update. That gives the team time to correct a risk before completion, when remedial work is usually more disruptive and expensive.
Official sources
Published 24 March 2026 · Last updated 8 September 2026
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