Extension Calculations

For Self-Build, Builders and Developers

SAP Calculations for Extensions

If your proposed extension contains large areas of glazing you will likely need a SAP Calculation/ Heat loss assessment to ensure your proposal meets energy efficiency requirements.

Do I need a SAP Calculation for my extension?

SAP calculations are used to calculate and assess the energy efficiency of houses. They are part of the government’s plan to reduce carbon emissions and tackle fuel poverty, and are needed to make sure new dwellings comply with Part L of the Building Regulations of England & Wales.
But some confusion can occur when it comes to extensions – do they need SAP calculations? The short answer is no, not all extensions require them. This depends on the amount of glazing within the proposed project.

Building Regulations require that, when planning an extension, areas of new glazing should account for no more than 25% of the new floor area (plus an allowance for any existing windows that are being removed). This is to maximise energy efficiency as glazing and openings lose more heat than walls and roofs.

Recent trends towards extending houses to add new, light airy spaces with full width bi-fold doors and skylights, many extension designs will propose more than the 25% allowance, and therefore will not comply against this rule. This is where SAP calculations are required.

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