How to Pass SAP: The Practical Checklist for Builders

Most SAP failures are avoidable. They usually come from small changes during the build that were never fed back into the calculation, or from assumptions at design stage that don’t match what happens on site. When the SAP model and the real construction drift apart, compliance becomes harder and the fixes more expensive.

This checklist is a straightforward way to keep your project on track. Follow it from the first day on site through to completion and you avoid the usual late-stage surprises.

Start with a realistic Design SAP

A SAP calculation is only as reliable as the information used to create it. Many projects fail because the design stage assumptions were optimistic. If the airtightness target is too low, or the glazing performance is better on paper than the products actually ordered, the design will pass comfortably but the as built version will fall short.

The Design SAP should be built around what you plan to install, not what looks good in a spreadsheet. If you need clarity on this stage, our design stage SAP guidance will help you avoid starting the build with unrealistic targets.

Confirm the insulation specification early

Insulation is one of the biggest drivers of heat loss and the most common area where plans and real-world construction begin to diverge. If the specified thickness doesn’t physically fit around steelwork, or the actual product on site has a higher thermal conductivity than the one modelled, the margin can disappear quickly.

The simplest way to avoid this is to check the insulation spec against the real build-up before any boards are ordered. If something needs changing, update the SAP model early.

Keep continuity at thermal junctions

Thermal bridging has become a far bigger part of SAP under the 2021 regulations. Junctions around openings, intermediate floors, lintels and eaves all influence the final Y-value. If the wrong detail is used on site or the photographic evidence isn’t clear, SAP defaults usually have to be applied. Those defaults are harsh and can turn a comfortable pass into a fail.

Follow the detail the design was based on and get photos before the area is covered. A few minutes of photography saves a lot of trouble later.

Treat airtightness as a planned task

Airtightness isn’t achieved by accident. It needs the same level of attention as insulation. Agree the air barrier line before plasterboard goes on, seal the penetrations early and avoid relying on last-minute mastic work.

A pre-test during second fix is a simple way to find the big leakage points while you still have access. Leaving the test until the final week of the build is where most failures occur.

Match the heating and ventilation system to the design

Heating and ventilation upgrades often happen late in a project. A boiler gets swapped for a different model, MVHR is replaced with intermittent extract, or the homeowner chooses a cheaper control system. These changes alter the SAP result immediately.

If something changes, tell the assessor at that point. Updating the SAP model takes minutes. Fixing a failure at completion is far harder.

Use the right glazing

Windows and doors are another common drift point. Many SAP designs assume a U value based on a proposed supplier, but the order placed later might be weaker. Even a small difference can move the SAP result more than expected.

Check the datasheets before ordering. If the numbers vary, update the calculation before they are installed.

Record evidence throughout the build

Part L requires photographic evidence of insulation, thermal junctions and key parts of the construction. If the photos aren’t taken at the right time, the assessor may have to use default values. Defaults almost always reduce performance.

A simple folder with the date, plot number and location is enough. Builders who keep evidence as they go rarely have SAP problems at the end.

Keep SAP updated when something changes

Most SAP failures come from projects where the design and the build drift apart and no one updates the model. SAP should be treated as a working document. If a product changes, a detail changes or the build has to be adjusted to suit the site, let the assessor know.

This small step prevents almost every late-stage fail.

Before completion

As soon as the building envelope is ready, arrange the air test. Once the test passes and the house is finished, the assessor can complete the as built SAP, generate the final BREL report and lodge the EPC. Building Control will not sign off the job until all three are provided.

If you reach this point and find the numbers are falling short, our as built SAP support can help you identify what is causing the gap and the simplest way to bring the project back into compliance.

Final thoughts

Passing SAP isn’t complicated when the design stage assumptions match the way the home is actually built. Most problems appear when insulation, glazing, airtightness and heating systems change without the SAP model being kept up to date. Builders who treat SAP as part of the build, rather than something done at the end, tend to have the smoothest sign-offs.

If you want help keeping the SAP process simple and predictable, I can guide you through every step from design to completion.