SAP Calculations for Extensions
When an extension's glazing goes past the standard allowance, the simple compliance route fails — and the design's future rests on a SAP calculation. We model the whole dwelling and show building control the extension performs no worse than a compliant one, keeping the glass in the design.
The 25% rule
Why glass-heavy designs fail the simple route
Most extensions never see a SAP calculation. They comply by the elemental route: the new walls, floor and roof meet the standard U-values, and the glazed area stays within the normal allowance — broadly 25% of the extension's new floor area, plus an allowance for any existing windows and doors the extension covers over. Building control checks the numbers, and that's that.
The designs people actually want, though, are exactly the ones that break the allowance. A kitchen extension wrapped in bifold doors, a garden room with a glazed gable, a rear addition under a run of rooflights — glass is the point of the project, and it overshoots 25% almost immediately. At that moment the elemental route is closed, but the design isn't dead: Part L allows a whole-dwelling SAP comparison instead.
The comparison building control accepts: the dwelling plus its proposed extension is modelled against the same dwelling plus a notional extension built exactly to the standard recipe. If the proposed version performs no worse, the design complies — glazing and all.
Designs this route rescues
- Kitchen extensions with bifold or sliding door walls
- Garden rooms and orangeries built around glazing
- Wraparound extensions with rooflights throughout
- Glazed gables and floor-to-ceiling picture windows
- Any design an architect has already drawn — and a builder has been told won't pass
The process
From drawings to a building control report
Send the architect's drawings
Plans and elevations of the extension, plus basic details of the existing dwelling — construction age, heating system and window types.
We check the glazing ratio
Sometimes a design only just overshoots, and we say so — if the elemental route can work with a minor tweak, that's the cheaper answer and you'll hear it from us first.
Whole-dwelling comparison
We model the dwelling with your proposed extension against the same dwelling with a notional compliant one, and see where the balance lands.
Compensatory measures if needed
Where the proposal falls short, we cost the options that pull it back — better fabric elsewhere, upgrades to the existing dwelling, smarter heating controls or renewables — and pick the cheapest set that passes.
Report issued
You get a report formatted for your building control body, demonstrating the extension performs no worse than the notional compliant design.
New build-ups need proving too. The extension's walls, floor and roof usually need U-value calculations to confirm the performance the SAP model assumes — we do those in-house from the same drawings, so one enquiry covers both.
Timing
Do this before the builder prices the job
The worst time to discover a glazing problem is after the order for the bifolds has gone in. Run the SAP comparison while the scheme is still on paper and the outcome shapes the specification instead of fighting it — the glazing stays, and any compensation is designed in from the start rather than bolted on under deadline pressure.
If your project is bigger than an extension — a full change of use, or a new dwelling in the garden — the SAP requirements differ, and we cover those routes separately. Whatever the project, send the drawings through our contact page and we'll confirm which assessment it actually needs before anything is priced.
What to have ready
- Scaled plans and elevations of the extension
- Window, door and rooflight sizes — or the schedule if drawn
- Intended wall, floor and roof build-ups
- Age and heating system of the existing dwelling
Common questions
Extension SAP questions, answered
How much does an extension SAP calculation cost?
It depends on the size of the extension, how much glazing the design carries and how complete the architect's drawings are. Send the drawings and a note of the intended glazing via the quote form and we'll reply with a fixed quote, usually the same working day.
Does my extension need a SAP calculation at all?
Most don't. If the new elements meet the standard fabric U-values and the glazed area stays within the normal allowance — broadly 25% of the extension's floor area, plus an allowance for any existing windows and doors the extension covers over — the simple route applies and no SAP is needed. It's when the design goes past that allowance that a SAP calculation becomes the way through building control.
What counts towards the 25% glazing figure?
The glazed area of the new windows, rooflights and glazed doors in the extension, measured against the extension's new floor area — with credit given for the area of any existing openings the extension builds over. Bifold doors, a run of rooflights and a glazed gable add up faster than most designs expect, which is why so many attractive extensions overshoot the allowance.
What if the extension fails even the whole-dwelling comparison?
Then we work through compensatory measures: better-than-minimum fabric elsewhere in the design, upgrades to the existing dwelling, improved heating controls, or renewables. The model shows exactly how much each measure claws back, so you keep the glazing and add the cheapest compensation that gets the comparison over the line.
Will the extension mean I need a new EPC?
Building an extension doesn't itself trigger an EPC — that requirement bites when the property is next sold or let. But if a sale is on the horizon, it's worth booking a fresh assessment once the work is done so the certificate reflects the improved property. See our domestic EPC service for that.