SAP calculations explained: design stage to as-built

What SAP is testing

SAP does not score a home in isolation. It builds a notional dwelling of the same size and shape with a defined specification, derives targets from it, and then asks whether your dwelling beats them. Under Part L 2021 that means the Dwelling Emission Rate must not exceed the Target Emission Rate, the Dwelling Primary Energy Rate must not exceed the Target Primary Energy Rate, and the Dwelling Fabric Energy Efficiency must not exceed the Target Fabric Energy Efficiency. Alongside those three tests, limiting fabric and services standards act as a floor: you cannot trade away a poor wall U-value entirely by adding renewables.

What your assessor needs from you

A design-stage SAP model is only as good as the information behind it. The commonest cause of delay is not a difficult building; it is a set of drawings that do not yet say what the walls are made of. Everything below can be provisional, but it needs to be stated rather than assumed.

Design stage: getting it right before anything is built

The design-stage calculation and BREL report are submitted to the building control body before work starts. That timing is the whole point: it is the last moment where a change costs a line on a drawing rather than a variation on site. If the first run fails, the fix is usually one of three things — reduce or re-orientate glazing, improve the thermal bridging assumptions with better junction details, or improve the heating and hot water specification. We would rather send you two options with costs attached than quietly pick the one that makes the model pass.

During construction

Between design and completion, the model has to keep pace with reality. Substituted windows, a different heat pump, a change of insulation product or a junction detail built differently all move the numbers. The cheap version of this is a short note to your assessor at the point of substitution; the expensive version is discovering it at the as-built stage when the plot is due to be handed over.

As-built and hand-over

At completion the calculation is re-run against what was actually built and installed, and it is supported by air tightness test results, commissioning records for the fixed building services, and photographic evidence of key construction details. The on-construction EPC for each new dwelling is produced from that as-built calculation. Building control will normally want the whole package together, so the plots that sail through are the ones where the evidence was collected as the work happened rather than reconstructed afterwards.

Where SAP ends and planning begins

A SAP calculation demonstrates compliance with Building Regulations. It is not the same exercise as a planning energy statement, and in London the two are assessed separately: a London Plan energy strategy is judged against Policy SI 2 by the planning authority, while the Part L calculation goes to building control. If your consent carries an energy condition, tell your assessor at the outset, because a scheme that satisfies Part L does not automatically satisfy a planning target set above it.