A SAP calculation — Standard Assessment Procedure — is the government-defined energy calc that demonstrates a new dwelling, a conversion, or in some cases an extension, complies with Part L of the Building Regulations. For a straightforward extension using the simplest compliance route, expect £95–£250. For a full whole-house SAP calculation — the route most new dwellings and conversions need, and the one some extensions choose deliberately — expect £150–£425 depending on floor area, usually covering the design-stage calculation, the as-built calculation once the work is finished, and the completion Energy Performance Certificate as one bundled fee.
But it depends on which route your project actually needs, and that's the part quotes rarely explain clearly. Not every extension needs a SAP calculation at all; plenty comply through a much simpler elemental check. New dwellings and conversions creating a habitable unit almost always need the full process, both stages, no way round it.
I should be straight about where I sit here: I'm a domestic energy assessor, but the accreditation I hold — covering RdSAP EPCs on existing homes — is a different qualification from the On Construction Domestic Energy Assessor (OCDEA) status a full new-build SAP calculation requires. I don't produce these myself. What follows is written because this topic comes up constantly in my own work — on extension and conversion jobs I survey, and on the EPC side it's most often confused with — not because I'm selling it.
The short answer
Figures below were checked on 2 September 2026 against current UK SAP assessor pricing from several providers, cross-referenced against each other. As with the rest of this series, quotes in London and the South East — my own patch included — commonly trend above the England-wide average for a comparable job.
| Route | Typical cost (2026) | What's included |
|---|---|---|
| Extension — elemental U-value check | £95–£125 | Confirms new walls, roof and floor each meet the required U-value; no whole-house modelling |
| Extension — whole-house SAP trade-off | £150–£425 | Models the whole dwelling to allow larger glazing or a weaker element offset elsewhere |
| New build or conversion, small–medium (<150m²) | £275–£350 | Design-stage SAP, as-built SAP and completion EPC bundled |
| New build or conversion, large (150–250m²+) | £350–£425+ | As above, plus longer modelling for a bigger, more complex dwelling |
Multi-unit developments usually price per plot rather than per the single-dwelling figures above, with a meaningful discount for volume once an assessor is modelling several similar units on one site. If a provider quotes a single figure without saying which stage or stages it covers, ask — a design-stage-only quote and a full design-plus-as-built-plus-EPC quote can look similar in price but aren't the same product.
What drives the price
Which route the project needs. A basic elemental U-value check for an extension is materially cheaper than a whole-house SAP calculation, because it only checks the new elements against a fixed standard rather than modelling the whole dwelling's energy balance.
Floor area. Bigger dwellings mean more rooms, more junctions and more input data to model, which is why new-build pricing is almost always tiered by size — sub-80m², 81–150m², 151–250m² and above, roughly.
Design complexity. Extensive glazing, multiple heating systems, unusual junctions or a mix of construction types all add modelling time, and each is a reason a project might be quoted at the top of its bracket rather than the bottom.
One stage or two. A design-stage calculation on its own, submitted to support a Building Control application before work starts, is one fee. Coming back afterwards for the as-built calculation and completion EPC is either bundled into that fee or charged separately — commonly a smaller top-up if the same assessor did the design stage.
Revisions. If the completed air-pressure test comes back worse than assumed at design stage, or the actual products installed differ from what was modelled, the as-built calculation needs rerunning to match reality before Building Control will sign off — and a mid-project specification change can trigger the same thing.
Volume. Multiple similar units on one development site cost meaningfully less per plot than a single one-off dwelling, because much of the modelling carries across.
Do you actually need one?
For a new dwelling or a conversion that creates a new dwelling, yes, without exception — both the design-stage and as-built calculations are a Building Control requirement, and the completion EPC that comes out of the as-built stage is a legal requirement before the property can be sold or let.
For an extension, it's genuinely conditional. Most modest single-storey and two-storey extensions comply through the elemental method: each new wall, roof, floor and window simply has to meet a fixed U-value on its own, checked without modelling the house as a whole. That route doesn't need a full SAP calculation at all, just the simpler check. A whole-house SAP calculation becomes the better option — sometimes the only realistic one — where the design doesn't meet the elemental standard on its own merits, most commonly because the glazing is large: extensive bifold doors or roof lanterns can push a scheme past the elemental limits, and a SAP trade-off lets you offset that against better insulation or a more efficient heating system elsewhere in the calculation. If your extension is straightforward with unremarkable glazing, ask your builder or architect directly whether Building Control actually needs a SAP calculation before paying for one — it's a legitimate question, and the honest answer is often no.
SAP, RdSAP and the EPC you might already have
These get mixed up constantly, understandably, because the acronyms and the underlying methodology overlap. RdSAP — Reduced Data SAP — is what produces the EPC on an existing home: an assessor visits, records what's observable, and applies conservative default assumptions where it isn't. It's the certificate I produce on survey and remortgage jobs. A SAP calculation for a new dwelling or extension is a different tool entirely: a design-stage energy model built from drawings and specifications, followed by an as-built version once the work is finished, and it requires the separate On Construction Domestic Energy Assessor (OCDEA) accreditation — a standard domestic EPC assessor's qualification doesn't automatically cover it. If a homeowner extending their property asks me for "the EPC for the extension," what they usually need first is the SAP calculation for Building Control, with a completion EPC as its by-product; the RdSAP EPC I do is a different document for a different purpose, and won't satisfy a Building Control requirement on its own.
The Future Homes Standard and what's actually changing
The Future Homes and Buildings Standard was published on 24 March 2026. The regulations come into force on 24 March 2027, with a twelve-month transition period after that — meaning full mandatory compliance for new homes and buildings sits effectively at March 2028, and a project already under construction can complete under the Part L version in force when it started. At launch, the approved methodology is SAP 10.3, not the long-discussed Home Energy Model (HEM); HEM is expected to become available as an alternative compliance route roughly three months after publication, but it isn't the sole or mandatory method. In practice, that means anyone starting a new build, conversion or extension now still needs a SAP calculation under broadly familiar rules — the sensible question to ask your assessor is simply which SAP version they're quoting against, since 10.3 carries tighter fabric and heating assumptions than the version it replaces.
What I'd watch for in a quote
- Confirm which route is being quoted — a simple elemental U-value check, or a full whole-house SAP calculation — before comparing two quotes that look similar in price.
- Check the accreditation is OCDEA specifically, not just a standard domestic EPC (DEA) qualification, for any new-build or conversion SAP calculation.
- Ask whether the as-built stage and completion EPC are bundled in, or a separate fee once the build is finished — and if it's a separate fee, get that figure now rather than at handover.
- Confirm which SAP version the assessor is using, particularly for anything that might complete after the Future Homes Standard's March 2027 force date.
- Ask what a revision costs if the air-pressure test or the as-installed specification differs from what was modelled at design stage — this is a common, and commonly unbudgeted, extra.
- Get a fixed fee against the drawings you actually have, not a verbal estimate, and confirm what a design change mid-project would cost to rerun.
Questions I get asked
How much does a SAP calculation cost in 2026? It depends on the route. A simple U-value or reference-method check for an extension typically runs £95–£250. Opting for a whole-house SAP trade-off on an extension, or a SAP calculation for a new dwelling or conversion, typically costs £150–£425 depending on floor area, usually covering the design-stage calculation, the as-built calculation and the completion EPC as one bundled fee.
Does every extension need a SAP calculation? No. Most modest single-storey extensions comply through the simpler elemental method — meeting set U-values for each new wall, roof and floor without any whole-house modelling. A SAP calculation only becomes necessary if you want the flexibility to trade off, for example larger glazing against better insulation elsewhere, or if the extension is large or complex enough that Building Control asks for it directly.
Is a SAP calculation the same as the EPC I already have? No, and this is where most confusion starts. An EPC on an existing home is produced under RdSAP, using observed and assumed data about a building that already exists. A SAP calculation for a new dwelling or extension is a design tool, modelled from drawings and specifications before and after the work is built, and it requires a separate On Construction Domestic Energy Assessor (OCDEA) qualification that a standard domestic EPC assessor doesn't automatically hold.
Will the Future Homes Standard change what a SAP calculation costs? Not immediately. The Future Homes and Buildings Standard was published on 24 March 2026 and the regulations come into force on 24 March 2027, with a twelve-month transition meaning full compliance is effectively required from March 2028. SAP 10.3 is the assessment methodology at launch, not the Home Energy Model, so a SAP calculation is still what most projects starting now will need — just check which SAP version your assessor is quoting against.
I don't produce new-build SAP calculations myself — that's the OCDEA assessor's job — but if you're planning a single-storey extension, a garage conversion or a loft conversion and want a straight read on the existing building before you commit, or an EPC on the rest of the property, that's where I can help. Call 07946 618203 or get in touch and I'll come back the same day.
Small print. This guide is general information, not advice on a specific project, and it doesn't replace a written quote from a qualified OCDEA assessor for your own drawings. I'm not the assessor for this particular service, as noted above; the pricing here is checked against the wider market. Prices were researched and correct to the best of my knowledge on 2 September 2026; costs and regulations move, and quotes vary by region, project size and route. Always get at least three written quotes and confirm with Building Control which compliance route your project actually needs.