SAP Calculations

A SAP (Standard Assessment Procedure) calculation is the full, design-stage methodology used to demonstrate Part L compliance for new build homes, conversions and certain extensions. It’s a different exercise to the RdSAP survey a Domestic Energy Assessor carries out to renew an expired EPC on an existing rental property – RdSAP works from a site survey of the property as it stands, using standardised assumptions, whereas full SAP is built from your actual design drawings and specifications before a single brick is laid. If you’re a landlord needing an EPC renewal on an existing home, you’ll need an RdSAP assessment rather than a full SAP calculation.

Often, yes. A change of use – converting a barn, garage or commercial unit to residential – typically requires a full SAP calculation to demonstrate Part L compliance, since it’s treated similarly to a new dwelling. Extensions don’t always trigger a SAP requirement, but a heavily glazed extension can push the whole property’s fabric performance outside what Building Control will accept without one, particularly where the extra glazing risks tipping the design into an over-glazing check under Part L. If you’re planning a conversion or a highly glazed extension, it’s worth checking early rather than assuming standard Building Regulations compliance routes will cover it.

For a new build or conversion, we’ll need scaled architectural drawings (plans, elevations and sections), glazing specifications (U-values and areas), proposed heating system and hot water details, and details of insulation specifications for the walls, floor and roof. The earlier and more complete this information is, the more accurate the calculation – and the less likely you are to need a revision once construction is underway.

As early as possible – ideally at the planning or design stage, so the results can inform decisions on fabric specification, glazing and heating systems before construction starts, rather than after. Making a change at design stage is far cheaper than retrofitting a fix once a property is built and failing to meet its target.

A failed initial calculation isn’t the end of the process – it’s the reason to run one at design stage rather than at completion. Where a design doesn’t meet target on the first pass, we work through the fabric and services specification to find a cost-effective fix: improving insulation U-values, adjusting glazing, upgrading heating efficiency or adding a modest amount of solar PV, rather than defaulting to a structural redesign. Catching this at the design stage, not after construction, is exactly what a SAP calculation is for.