Design-Stage and As-Built SAP: What Changes

Design-Stage and As-Built SAP: What Changes

Every new home needs two SAP calculations. One before work starts, using the design. One when the home is finished, using what was actually built. The targets stay the same. The inputs change, and those changes decide whether the home passes at the end.

Two calculations, one set of targets

Approved Document L says the calculations must be carried out at the design stage and again when work is complete. Each time, the home is compared with a notional home of the same size and shape. Under Part L 2021 there are three targets: the emission rate, the primary energy rate and the fabric energy efficiency rate. The home's figures must not be worse than the targets at either stage.

A pass at design stage is not the end of it. It is the first half.

Design stage: before work starts

The Building Regulations require the person carrying out the work to send building control a notice no later than the day before work starts. It gives the target rates, the rates for the home as designed and a list of the specifications used in the calculation. On a higher-risk building, it goes in with the building control approval application instead.

The design-stage BREL report covers this. From the same calculation we also give you a predicted energy assessment, so you can market the homes off plan.

At this stage the air permeability is a design value, not a test result. Set it too low and the home passes on paper, then fails once the real figure goes in.

As-built: within five days of completion

Not later than five days after the work is complete, building control needs a second notice. It gives the rates for the home as constructed, says whether it was built to the original list of specifications and lists any changes. Building control can also accept a certificate from an accredited energy assessor.

The as-built calculation must include any changes made during construction and the measured air permeability from the air test. Approved Document L lets building control accept the completion-stage BREL report together with photographic evidence, provided for each individual home.

The on-construction EPC comes from the as-built SAP. It must be given to the owner, with a notice to the local authority, no later than five days after the work is completed. We lodge it for you.

What usually changes between the two

• The air test result. The measured figure is worse than the design value. This is the most common reason an as-built SAP goes backwards.
• Product swaps. Different windows, insulation, heat pump, boiler or ventilation unit. Approved Document L says a revised specification should go into the SAP and the BREL report.
• Junctions. Details built differently from the drawings, or no photographs to show how they were built.
• Solar panels. Fewer panels, a different layout or a different orientation from the design.
• The layout. Changes to plans, glazing or rooflights after the design SAP was issued.

How to keep the as-built result in line

• Use a realistic air permeability at design stage. We set one the builder can hit, so the result holds after the test.
• Tell us before you swap a product, not after it is fitted. We rerun the SAP and tell you what it does to the result.
• Take photographs of the insulation and junctions before they are covered, for every home. Approved Document L expects an on-site check before details are concealed.
• Keep the paperwork as you go: the air test certificate, commissioning certificates for heating and ventilation, MCS certificates for any renewables and the product data for what was installed.

Part L 2021 or the Future Homes Standard?

The new Part L comes into force on 24 March 2027. A building can stay on the 2021 rules if its building notice, initial notice or full plans went in before 24 March 2027 and work on it starts before 24 March 2028. It is judged building by building, not site by site. Higher-risk buildings have their own dates.

On a long build, some homes on the same site can end up on different rules. Send us your dates and we will confirm which rules, and which SAP version, apply to each home before the design SAP is run.

Questions we get asked

Someone else did the design SAP. Can you do the as-built? Yes. Send us the design-stage report, the drawings and details of what was installed. We check the model before the air test, so there are no surprises at the end.

The as-built SAP fails. What now? We find the lowest-cost fix. That might be calculated psi-values for the main junctions, a re-test after sealing, or more solar panels. Tell us as early as you can, because the options shrink once the home is finished.

Do we need a new design SAP if the specification changes on site? The design-stage notice stays as it is. The changes are listed with the as-built notice. We rerun the calculation when the specification changes, so you know where you stand before the home is finished.

How much does it cost? SAP calculations start from £100+VAT per home, including the U-value calculations and the EPC. That covers the design stage, a few reruns and the as-built. Turnaround is 3 working days once we have everything we need.

More: SAP calculations and EPCs, how much SAP calculations cost, air tightness testing, how to pass an air tightness test first time, PSI value calculations, the Home Energy Model and the Future Homes Standard and the latest industry updates.

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