SAP calculations for an extension start at from £225 with us. A partner assessor prepares them, and we coordinate them with the building regulations drawings and specification so the result and the build-up agree.
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Many extensions never need one. The honest first question is whether your design can meet Part L by checking each element against its limit, which costs nothing. The calculation is for the design that cannot, usually because it has a lot of glass.
When does an extension need a SAP calculation?
When the design does not meet the element limits and you want to keep it. Approved Document L Volume 1 sets maximum U-values for new extension elements: 0.15 for the roof, 0.18 for walls and 0.18 for the floor, 1.4 for windows and 2.2 for rooflights (Approved Document L). Windows, roof windows, rooflights and doors together must not exceed 25% of the extension's floor area, plus any openings it removes or covers (paragraph 10.8(d)).
A 20 sq m kitchen extension therefore allows 5 sq m of glazing. Bifold or sliding doors across the back use that up quickly, and that is the usual case where a calculation comes in. Our Part L guide works the arithmetic through.
What are the alternatives to a SAP calculation?
Two methods sit between the simple rules and the full calculation. Under the area-weighted method, an extension with more glazing is accepted where its average U-value is no worse than a compliant extension of the same size and shape (paragraph 10.9). Better insulation elsewhere pays for the extra glass.
The SAP method, in paragraph 10.10, compares the whole dwelling plus the extension with the dwelling plus a notional compliant extension. It is the most flexible route and the one that needs an assessor. We tell you which method fits your design before we order a calculation, because the area-weighted method may be enough.
Does the Future Homes Standard change this?
Largely not for extensions. The 2026 edition of Approved Document L takes effect on 24 March 2027 and leaves the four extension U-values unchanged, and the guidance on renewable electricity does not apply when a dwelling is extended (GOV.UK, building circular 01/2026). The whole-house targets are for new dwellings.
If your building control application is made near the changeover, check which edition applies to your project with your building control body.
What does it cost on top of the drawings?
Our SAP service starts at from £225. Our building regulations drawings are £545 drawings + £150 survey (£695 total), and our full extension package covers planning, building regulations and structure. A SAP calculation is an extra only when the design needs it, which is why it is priced separately and not folded into the others. Building control's own charge is paid to your council or registered approver.
Frequently asked questions
Is a SAP calculation the same as an EPC? No. A SAP calculation for building regulations models the design before it is built. An energy performance certificate rates an existing building. They use related methods, but they answer different questions.
Can I avoid the calculation by reducing the glazing? Often, yes: bring the glazing within 25% of the floor area and use the element limits. We can show what that costs you in the design.
Who prepares it? A partner assessor. We coordinate their result with our drawings and specification.
Is it needed for a loft conversion? Part L also applies to work on an existing roof, so a loft can need its own check. Ask us, and we check your design against the limits before suggesting a calculation.
