Two calculations, on a clock
Building Control requires the SAP at two stages of every new dwelling, and the regulations put a date on each.
Design stage — before work starts. We model the dwelling from your drawings and specification against the notional target dwelling. It passes when primary energy (DPER), carbon (DER) and fabric energy efficiency (DFEE) are all no worse than their targets and every element sits within the limiting standards. The target rates, the as-designed rates and the list of specifications the calculation depends on go to Building Control on the design-stage BREL report by the day before work begins.
As-built — within five days of completion. A new calculation, using the dwelling as actually constructed: the measured air test result, the make and model of the heating installed, the lighting, the ventilation, and every change from the design specification. The completion-stage BREL is signed by us and by the builder, accompanied by the photographic evidence, and notified to Building Control. We lodge the EPC from the as-built calculation — Building Control cannot issue the completion certificate without it.
What the dwelling is measured against
The notional dwelling is the same size and shape as yours but built to Approved Document L’s reference specification: walls 0.18, floors 0.13, roof 0.11, windows 1.2 W/m²K; an air permeability of 5; a gas boiler at 89.5% with a 55°C flow temperature; waste water heat recovery on every shower; 80 lumens-per-watt lighting; and solar PV at 40% of the ground floor area divided by 6.5. That PV assumption is the one that catches people: the target you have to beat already has panels on the roof.
The limiting standards are the other half. Nothing may be worse than a wall at 0.26, a floor at 0.18, a roof at 0.16, windows and doors at 1.6, rooflights at 2.2, or an air permeability of 8 — but a dwelling built to those backstops will fail the targets by a distance.
Where new builds fail, and what fixes it
From the base calculation, the fixes fall in a fairly reliable order of cost:
- Calculated junction Ψ-values. SAP’s default thermal-bridging values are deliberately punitive. Calculating the real value for your actual details — which our free ΨMonkey calculator does to BS EN ISO 10211 — is usually the cheapest single improvement, because it costs nothing on site. Building to Recognised Construction Details is the other route: published junction values, no calculation, provided the details are followed exactly on site.
- The air test target. Committing to 5 or better instead of the backstop, then building to it.
- Fabric. Another 25 mm in the cavity, a better window specification — cheap on paper, expensive after the order is placed.
- Heating. A heat pump against SAP 10.2’s electricity carbon factor of 0.136 kgCO₂/kWh, versus 0.210 for gas (0.086 and 0.214 under SAP 10.3 for the Future Homes Standard), moves the carbon and primary energy results a long way.
- PV. Matching the notional dwelling’s panels, or more.
We build the base calculation, and if it fails we do not just report that: we model the scenarios and recommend the combination that passes for the least money, based on what is practical for your build.
The specification list is the contract
The list of specifications notified at design stage is what the as-built calculation is checked against, and it is where new builds go wrong. A different insulation product, a cheaper glazing unit, a boiler swapped for the one in stock — each has to be reported at completion and re-run, and some turn a pass into a fail with the house already standing. Tell us before the substitution is ordered. We keep the model, so a re-run is a revision, not a restart.
The photographs are part of the same discipline. Since the 2021 edition every dwelling needs a set showing the insulation continuous and properly fitted at six stages — foundations and ground floor; each external wall type; each roof type; each opening type with its reveals and cavity closers; the airtightness details; and the services, including pipework insulation. Date-stamped and located, normally taken by the builder, reviewed by us against the specification before we sign the as-built BREL. Tell the site team on day one, because there is no way to photograph insulation that has been plastered over. The photographic evidence guide lists all thirteen details with the point on the programme to catch each one, and there is a free printable plot checklist.
How it works
It is a desktop study — no site visit or survey. Send us plans, sections and elevations (PDF, DWG or paper) plus a construction and services specification, provisional if that is all there is. We come back with a price and a turnaround, usually the same day, and stay on call through the build for the “does this still pass?” questions. Flats are modelled as a block so party walls, communal heating and shared elements are consistent, and each dwelling gets its own calculation and EPC.
In a hurry?
Ask about ASAP, our fast-track priority SAP service for tight deadlines.
Building across the 2027 change
Approved Document L 2026 takes effect on 24 March 2027. A dwelling with its building notice or full plans in before that date can still be built to the current targets provided work starts by 24 March 2028; otherwise the new edition applies — a heat pump and a tighter air permeability in the notional dwelling, and on-site renewables as a legal requirement under the new L3. SAP 10.3 is already the calculation engine either way. If your programme sits near the line, tell us and we will tell you which side of it you fall.
Common questions
When does the design-stage SAP have to be done?
Before work starts. The regulations require the target and as-designed rates and the list of specifications to be notified to Building Control by the day before work begins — reported on the design-stage BREL. In practice you want it earlier than that, while the specification can still change cheaply.
Why does a well-insulated new build still fail SAP?
Because it is compared with a notional dwelling that already has solar PV on the roof, waste water heat recovery on every shower, an air permeability of 5 and walls at 0.18. Fabric alone rarely beats that. The usual answers are PV, a heat pump, a tighter air test target, and calculated junction psi values instead of the default ones — in whichever combination is cheapest for your build.
What is the as-built SAP for?
To prove that what was built is what was approved. It uses the dwelling as constructed — the measured air test result, the heating actually installed, every change from the design specification — and it must be notified to Building Control within five days of completion on the completion-stage BREL, with the photographic evidence. The EPC is produced from it, and the completion certificate depends on the EPC.
Who takes the Part L photographs?
Normally the builder — there is no restriction on who does it. What matters is that every dwelling has its own set covering the six categories, date-stamped and located, taken while the insulation is still visible. The assessor reviews them against the specification before signing the as-built BREL. A plot without photographs cannot be signed off.
Can I change the specification during the build?
Yes, but every change has to be reported at the as-built stage and the calculation re-run, and some changes turn a pass into a fail. Tell us before the substitution is ordered and we will tell you whether it still passes. That is much cheaper than finding out at completion.
Does each flat in a block need its own SAP?
Yes. Each dwelling is assessed and has its own EPC, though the block is modelled together so that party walls, communal heating and shared elements are treated consistently. Mid-floor flats and top-floor flats can pass or fail differently from the same specification.
My project spans the 24 March 2027 change — which rules apply?
If the building notice or full plans application is submitted before 24 March 2027 and work starts on site by 24 March 2028, the dwelling can be built to the current 2021 targets. Miss either date and Approved Document L 2026 applies, with a heat pump and PV in the notional dwelling and on-site renewables as a legal requirement. Tell us the programme and we will tell you which side you fall on.