SAP calculations for new homes and conversions in London
A new dwelling, or a conversion that creates one, has to show building control that it meets the energy standard in Part L, and the evidence is a SAP calculation. An energy assessor we coordinate runs it from your drawings and specification, at design stage and again at completion, for a fixed £225.
Fixed fee from £225 · all prices
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What SAP is and what it is used for
The Standard Assessment Procedure is the government's methodology for estimating the energy performance of homes. GOV.UK says it has two main uses: to demonstrate compliance of new homes with Part L of the Building Regulations, and to generate Energy Performance Certificates for all homes. The current version is SAP 10.3.
A SAP calculation takes your drawings and specification and turns them into a number, so it can only be as accurate as the information it is given. That is why we send the assessor a coordinated set rather than a pile of PDFs: the window schedule, the wall build-ups and the heating decisions all have to agree with what will be built.
SAP Calculations
from £225
Every price is the full fixed fee you pay. No VAT is charged. Council application fees are paid to the council separately.
What you get
- A design-stage SAP calculation for the building control application
- Checks of your fabric, glazing, heating and ventilation against the notional dwelling
- A list of the specifications the building must be built to
- Advice on what to change if the first run fails
- An as-built SAP once work is complete, with the details the builder actually fitted
- The documents building control and your Energy Performance Certificate need
- Drawings and specification
Plans, sections, window schedule and the build-up of walls, roof and floor.
- Design-stage SAP
The assessor models the home and compares it with the notional dwelling.
- Adjust if needed
A little more insulation, a different glazing or a heat pump can turn a fail into a pass.
- Build to the list
The specification list is what the builder fits and building control inspects.
- As-built SAP
A final calculation at completion, with changes recorded, and the certificate you need.
Which projects need one
Approved Document L volume 1 says that a new dwelling must be built to a minimum standard of total energy performance, evaluated by comparing the actual dwelling against a theoretical notional dwelling, and that this must be done both at the design stage and when work is complete. That covers new houses and flats. A conversion is dealt with under the material change of use rules in regulation 6, and whether it needs a SAP depends on what the conversion creates, which we check against the guidance before you commit.
A house extension is different. The guidance for extensions sets fabric standards for new elements and a limit on glazing, with the total area of windows, roof windows, rooflights and doors in the extension not exceeding 25% of the floor area of the extension plus any openings lost. That route usually needs no SAP calculation, and where it does not, we say so and you do not pay for one.
GOV.UK says the Home Energy Model will replace SAP for the energy rating of dwellings, and that when the model has not yet launched, SAP 10.3 is the sole approved methodology, with the Home Energy Model to be introduced alongside it.
Design stage and as-built
There are two calculations because there are two moments building control cares about. The design-stage calculation goes in with the application and shows the building will pass if built as drawn. The as-built calculation is run at completion and reflects what was actually fitted: the boiler that arrived, the glazing that was substituted, the insulation thickness on site.
The Building Regulations require the person carrying out the work to give the council a notice, with the as-built figures and the specification, not later than five days after the work has been completed. We keep the design-stage specification list as the benchmark, so any substitution made on site can be rerun quickly.
What is changing, and when
The standards are moving. The Building Regulations etc. (Amendment) (England) Regulations 2026 introduce the Future Homes and Buildings Standards, with new versions of Approved Documents L and F, and the circular sent to councils says they come into force on 24 March 2027, with transitional provisions. We check which set of rules your application falls under before the assessor starts, because it decides which calculation is run.
The methodology is also changing. GOV.UK says the Home Energy Model will replace SAP for the energy rating of dwellings, and that when the model has not yet launched, SAP 10.3 is the sole approved methodology, with the Home Energy Model to be introduced alongside it. Until then, SAP 10.3 is what building control accepts, and we tell you if that changes during your project.
If the first run fails
A failing first run is normal and cheap to fix on paper. The usual levers are a little more insulation in the roof or floor, a different glazing specification, an air tightness target, or a heat pump in place of a boiler. Each has a cost on site, so the assessor and we compare options before anything changes on the drawings.
Changing the specification after the application is a nuisance, which is why we prefer the SAP at the start of the building regulations drawings, not the end. See our building regulations drawings, which are prepared alongside the calculation on new builds and conversions.
Who does the work, and the fee
The calculation is carried out by an accredited energy assessor that we coordinate, not by us. What we provide is the drawing set, the specification and the checks that the calculation matches what you will build. If the assessor's output asks for a change, we amend the drawings.
The fixed fee is £225, the total you pay, with no VAT charged. It covers the design-stage calculation for a typical new home or conversion. Larger schemes, multiple dwellings and an as-built re-run after substantial site changes are quoted before we start. Send the plans and the address, and we will say which route applies.
Sources
Every figure on this page, where it comes from and a link to the original.
| Figure | Source | Link |
|---|---|---|
| Part L | GOV.UK, Standard Assessment Procedure: the methodology has 2 main uses (source wording: "to demonstrate compliance of new homes with Part L of the Building Regulations") | www.gov.uk |
| SAP 10.3 | GOV.UK, Standard Assessment Procedure (update 8 June 2026): (source wording: "SAP 10.3 is currently available as the sole approved methodology") | www.gov.uk |
| Home Energy Model will replace SAP | GOV.UK, Standard Assessment Procedure: (source wording: "The Home Energy Model will replace the SAP for the energy rating of dwellings") | www.gov.uk |
| design stage | Approved Document L Volume 1 (2021 edition incorporating 2023 amendments) para 1.1: (source wording: "This must be carried out both at the design stage and when work is complete") | assets.publishing.service.gov.uk |
| 25% | Approved Document L Volume 1 para 10.7(d)(i): the total area of windows, roof windows, rooflights and doors in extensions should not exceed (source wording: "25% of the floor area of the extension") | assets.publishing.service.gov.uk |
| five days | Building Regulations 2010 reg 26/27 as reproduced in Approved Document L Volume 1: (source wording: "Not later than five days after the work has been completed, the person carrying out the work shall give the local authority a notice") | assets.publishing.service.gov.uk |
| 24 March 2027 | MHCLG Building Circular letter 01/2026, 24 March 2026: (source wording: "The Regulations come into force on 24 March 2027") | www.gov.uk |