Design SAP vs As Built SAP: What Builders and Designers Need to Know

Most builders, architects and homeowners know they need a SAP calculation for Building Control, but the process involves two separate assessments. A Design SAP is produced before construction begins, and an As Built SAP is completed once the dwelling is finished. Each one is required at a different stage of the project and each serves a different role in meeting Part L.

Understanding the difference early removes most of the stress around compliance and prevents late changes at the end of the build.

When a Design SAP Is Required

A Design SAP is required before any building work begins. Building Control must be satisfied that the proposed dwelling will meet Part L, and they cannot approve commencement without evidence of compliance. The Design SAP provides this evidence and produces the initial BREL or BRWL report needed at submission stage.

At this point the assessment is based on drawings, specifications and assumptions, because the building does not yet exist. Airtightness, insulation performance, heating systems and junction details are all entered as intended values. The Design SAP confirms that the planned construction can meet the energy and carbon targets set out in the regulations.

If the assumptions are unrealistic, the project may pass on paper but struggle later. Early clarity on specifications prevents this.

When an As Built SAP Is Required

An As Built SAP is required once the dwelling is complete. This version must reflect the actual construction, not the planned one. Every assumption from the design stage is replaced with real evidence.

The As Built SAP uses:

• The measured air permeability result
• Photographs of thermal elements and junctions
• Datasheets for glazing, insulation and heating systems
• The final construction build-up for each element

This assessment produces the final BREL or BRWL report that Building Control must receive before sign-off.

The EPC must also be lodged within 7 days of completion, which means the As Built SAP must be completed immediately when the build is finished. Since an EPC cannot be lodged without a valid As Built SAP, the timing is effectively fixed by regulation.

Why the Two Assessments Differ

The Design SAP shows what the dwelling is expected to achieve. The As Built SAP shows what it actually achieved. The two often differ because of small changes on site. Substituted glazing, alternative insulation, adjustments to roof thickness, different heating systems or a higher air test result can all shift the numbers enough to cause problems at completion.

These changes seem minor during construction but can push the DER, DPER or TFEE above the target values.

The Role of Evidence in the As Built SAP

Part L requires photographic evidence of key construction stages. This includes insulation installation, thermal junctions, services and any area where performance relies on workmanship. Without this evidence, default values may need to be applied, which usually makes compliance harder.

The air permeability test is one of the biggest influences on the final result. The measured value replaces the assumed design stage figure, and a result higher than expected can quickly reduce the compliance margin.

Heating and ventilation systems must be recorded using the correct PCDB entries. If performance data cannot be confirmed, SAP defaults may apply.

Keeping the Two Assessments Aligned

The simplest way to avoid late failures is to treat SAP as a live document. If a product change or design adjustment happens on site, updating the model at that point keeps the Design SAP and As Built SAP aligned. Most issues that cause problems at completion would have been straightforward to resolve much earlier in the build.

Conclusion

A Design SAP confirms that a dwelling can comply with Part L before work starts. An As Built SAP proves that it does comply once construction is complete. Both are required under the regulations and both must be accurate for Building Control to sign off the project.

If you want to avoid late design changes or you need a clear process for keeping your SAP assessments aligned throughout the build, I can review your plans and guide you through both stages.

Need support with Design SAP, As Built SAP or the full compliance process? You can start here with our SAP Assessment service.