If you already know what a SAP calculation is and why your project needs one, the next question is usually what the process actually involves – and why people talk about a “Design SAP” and an “As-Built SAP” as if they’re different things. They are: a SAP calculation isn’t a single event but runs in stages from initial design through to the completed building, and understanding the difference matters, because a design that passes on paper still needs to prove itself once the building exists.
In this blog you’ll learn:
- The difference between a Design SAP and an As-Built SAP, and why both are needed
- The four stages a SAP assessment actually goes through, from design draft to completion
- What information is needed at each stage and who provides it
Design SAP vs As-Built SAP
A Design SAP is carried out during the design stage, using plans, specifications and proposed materials – insulation levels, heating systems, renewable technology, and so on – to estimate compliance and energy performance before construction begins. It’s the stage where problems are cheapest to fix.
An As-Built SAP happens after construction is complete, once the actual materials and systems installed are known, including any deviations from the original design. It verifies that the completed building genuinely meets the standard it was designed to – not just that the drawings said it would.
Both stages are required for new-build compliance (this applies to full new-build assessments, referred to as SAP L1A; a change-of-use assessment is a SAP L1B), and the difference between the two figures is often the first sign that something didn’t get built quite as specified.
The Four Stages of a SAP Assessment
Stage 1: Design Draft.
Using the plans and drawings provided, the SAP assessor summarises the key numerical information – total floor area, living room floor area, heat loss floor and roof areas, window and door dimensions, storey heights – then calculates the performance of each thermal element as a U-value. The software checks these against Part L requirements, and the assessor can model different design variations if the initial specification doesn’t show compliance, advising the designer on where the shortfall is and how to close it.
Stage 2: Design Final.
Once the design is agreed – incorporating any changes needed to reach compliance – the finalised data is entered into the software, and the assessor produces a Predicted Energy Assessment (PEA) report (a predicted version of SAP) for submission to Building Control.
Stage 3: Built Draft.
After construction and an air tightness test, the client or designer provides the test result and details of any variations from the original specification. The assessor updates the SAP calculation accordingly and checks the completed dwelling still meets Part L – recommending remedial action if it doesn’t. At this stage the assessor also confirms the dwelling is registered on the government’s central property address database, arranging registration if it isn’t.
Stage 4: Built Final.
The assessor finalises the calculation and generates the EPC, based on the dwelling as actually built – which by law must be displayed when the property is marketed for sale. Building Control also requires a SAP worksheet report and SAP data input alongside this, and the assessor provides the EPC reference so the client can retrieve a copy from the government’s central registry.
What Information Is Needed, and When
Design-stage input requirements include floor plans, elevations, sections and site plans; insulation type and thickness; heating and hot water system specification; ventilation system type; renewable technology details; accredited construction details; and U-values for all openings.
Built-stage updates then layer in the actual air pressure test result and confirmation of what was genuinely installed, rather than what was originally specified.
What’s next?
The gap between a Design SAP and an As-Built SAP is exactly where problems tend to surface – a heating system substituted on site, insulation that didn’t go in quite as specified, an air test that came back tighter or looser than modelled. Catching that gap early, rather than at final sign-off, is the whole point of running the process in stages.
We carry out both Design SAP and As-Built SAP calculations, so if you need support at either stage – or both, end to end – our team can walk you through what’s needed.
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