What is a SAP calculation and what is it used for?

What is a SAP calculation?

A SAP calculation models how much energy a dwelling is expected to use for space heating, hot water, ventilation and fixed lighting, and how much carbon that energy use produces. It is based on the design of the home rather than how the occupants will actually live in it, so different dwellings can be compared on a consistent basis.

The current methodology used for new dwellings in England is SAP 10.3, part of the SAP 10 methodology. The calculation is carried out by an accredited SAP assessor using approved software.

When is a SAP calculation required?

A SAP calculation is typically needed for new-build dwellings and for building work that creates a new dwelling, such as converting a barn, commercial unit or larger house into flats. Some extensions may also use SAP where the design does not follow the standard elemental route, for example where glazing exceeds the usual allowance and compliance is shown by comparing the whole dwelling.

Building Control will normally expect a design-stage calculation before work starts and an as-built calculation on completion. Requirements can vary with the type of work, so it is worth checking early with your Building Control body or assessor.

What information goes into a SAP assessment?

The assessor builds a model of the dwelling from the drawings and specification. This usually includes:

• floor plans, sections and elevations, so floor areas, wall areas and heights can be measured
• the construction of walls, floors and roofs, and their U-values
• window and door sizes, orientation and performance
• the heating system, controls and hot water arrangement
• the ventilation strategy and design airtightness
• thermal bridging details at junctions
• any renewable or low-carbon technologies, such as heat pumps or solar PV

What does the assessment check?

For new dwellings, the calculated performance is compared against a notional dwelling of the same size and shape built to a reference specification. The design generally needs to meet targets for primary energy, carbon emissions and fabric energy efficiency, and individual elements and systems also need to meet minimum standards.

The output is a set of figures and reports that Building Control can review alongside the rest of the application.

Design-stage and as-built SAP

The design-stage SAP is completed before construction, using the intended specification. It shows whether the design is on track and highlights changes worth making while they are still easy to make.

The as-built SAP is completed at the end of the project and reflects what was actually installed, including the final airtightness result where applicable. If the specification changed during the build, the as-built calculation needs to reflect those changes.

SAP calculations and EPCs

For a new dwelling, the as-built SAP calculation is used to produce the on-construction Energy Performance Certificate (EPC). The EPC presents the energy rating in a form that buyers, tenants and lenders recognise, so the two are closely linked but serve different purposes.

What happens if the design does not comply?

If the first calculation does not meet the targets, the assessor can usually identify which changes will make the biggest difference. That might mean improving insulation, revisiting the glazing, choosing a more efficient heating system, improving airtightness or adding solar PV. Finding this out at design stage is far easier and cheaper than discovering it at completion.

Our price guide gives an idea of the cost for typical residential projects.