SAP Calculations:
Covering England and Wales

SAP calculations are the Standard Assessment Procedure calculations used to assess the energy performance of dwellings. In practice, they are used to show that a new home or newly created dwelling meets the relevant energy targets and to support the EPC route for that dwelling. They are the core Part L domestic calculation method, not just an admin extra at the end.

SAP is the approved methodology for producing an EPC for a newly constructed dwelling, while RdSAP is the reduced-data method used for existing dwellings. In practical terms, SAP uses detailed plans and specifications, whereas RdSAP uses an on-site survey plus conventions because much less is known about the existing home.

Yes. SAP remains the approved methodology for new dwellings, and government guidance says it is used to demonstrate compliance of new homes with Part L. Whether the project is a single self-build house or a multi-flat scheme, the domestic dwellings sit on the SAP side of the regime.

Yes. Official EPC guidance treats new dwellings as including new builds, conversions and change of use of existing properties. If the work creates a new dwelling unit, SAP is the correct domestic calculation route rather than the ordinary existing-house EPC method. That makes SAP calculations a core service on conversion-led residential projects.

No, not always. Straightforward extensions can often comply by following the standard elemental route in Approved Document L, but SAP-style calculations become useful where the design needs more flexibility, such as higher glazing or compensation elsewhere in the build-up. In other words, not every extension needs a full SAP trade-off, but plenty do.

Design-stage SAP is the SAP calculation based on the dwelling as designed before work starts. It is used to show the intended performance of the home and to support the first compliance submission to Building Control. In practice, this is the version you want locked down before site starts changing products and details under programme pressure.

As-built SAP is the SAP calculation based on the dwelling as actually constructed. It has to reflect specification changes made during the build and incorporate the measured air permeability. This is the version that matters at completion, because it shows whether the finished home still meets the required performance once theory has turned into site reality.

Yes. The final SAP calculation has to incorporate the measured air permeability. Approved Document L in England says the dwelling primary energy rate, dwelling emission rate and fabric energy efficiency rate must be recalculated using the measured air permeability, and the Welsh guidance says the same basic thing. That is why air testing and SAP should never be treated as separate workstreams.

Yes. Government has updated the approved methodology from SAP 10.2 to SAP 10.3, and its SAP guidance says SAP 10.3 will initially be the sole approved methodology while the Home Energy Model is delayed. At the same time, the wider 2026 Future Homes Standard regulations generally take effect from 24 March 2027, so project timing and transitional arrangements still matter.

Get the assessor involved early, complete the design-stage SAP before works begin, keep the specification under control, and update changes as they happen instead of trying to reconstruct them at the end. The official England and Wales process is built around design-stage and as-built reporting for exactly this reason. The projects that “get it right first time” are usually the ones that treat SAP as a live compliance tool, not a last-week certificate request.