SAP (Standard Assessment Procedure) is the UK Government’s approved methodology for assessing the energy performance of dwellings. While full SAP calculations are generally used for new builds, extensions and certain conversions must also demonstrate compliance with Part L1B of the Building Regulations (England & Wales), which governs existing dwellings.

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In the context of extensions, SAP calculations are used to assess the carbon emissions and thermal performance to ensure that the proposed works do not worsen the overall energy efficiency of the property.

When SAP Calculations Are Required

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SAP calculations for extensions may be required when:

• The extension has a large amount of glazing (more than 25% of floor area).

• Multiple alterations are being made, including changes to the existing envelope or services.

• There is a need to justify performance beyond elemental U-value targets.

• The extension is thermally separate but has conditioned space.

 

Calculation Approaches

There are typically two routes to compliance:

1. Elemental Method

   • Meet or exceed prescribed U-values (walls, roofs, floors, windows).

   • Glazing should not exceed 25% of the floor area unless offset with improvements elsewhere.

   • Suitable for straightforward designs.

2. Whole Dwelling Method (SAP Comparison)

   • SAP calculation of the dwelling before and after the extension.

   • Demonstrate that the dwelling “as extended” performs no worse than the original.

   • Allows flexibility, e.g. increased glazing can be offset by:

     • Installing or upgrading insulation.

     • Improving heating system efficiency.

     • Adding renewable technologies (e.g., solar PV).

     • Improving airtightness or controls.