SAP calculations for Part L cost from £225 at Fixed Fee Plans. A partner assessor runs the energy calculation from our drawings and specification, and we coordinate it with the construction drawings so building control reads one story, not two.
Plenty of extensions need no calculation at all. The first thing we do is check, because selling you a SAP you do not need helps nobody. When your project does need one, it is usually because it is a newly built home, or because the design leans on more glass than the simple rule allows.
When does a project need a SAP calculation?
New dwellings always do. A newly built house or block of flats is judged against a notional dwelling of the same size and shape, at design stage and again when the work is complete, as paragraph 1.1 of the 2026 Approved Document L describes. The document also records that, for a new dwelling, two methodologies are approved: the Home Energy Model and the Standard Assessment Procedure.
For extensions, the elemental route usually works: new walls, roof, floor and windows at or below the limiting U-values, and glazing within 25% of the extension's floor area, plus the area of any opening it removes or covers. That limit is paragraph 10.7(d) of the 2021 edition. Past it, you can use the area-weighted method, or SAP, which paragraph 10.10 of the 2026 edition allows as an alternative: the whole dwelling plus extension must do no worse than the dwelling with a notional compliant extension. Our Part L guide works through the figures.
Why use SAP if the simple route works?
You do not, unless the design needs it. SAP earns its place when the scheme has more glazing than 25% allows: a glass-heavy rear extension, a large lantern, a set of bifold doors that is the reason you are building. It lets the result be met by better insulation or a better heating system elsewhere, where the elemental method would forbid the glass.
That is a design question, so we run it on paper first. The assessor tests the options, and we show you which change moves the result most for the least spend: thicker wall insulation, a heat pump, a smaller rooflight. A calculation that fails at design stage costs a few hours of re-work. One that fails at completion costs a wall.
What happens at completion?
A second report. The 2026 document says a design-stage BREL report is produced before works begin, listing the target and the dwelling's primary energy rate, emission rate and fabric energy efficiency rate with the supporting specification. An as-built report then repeats the figures with any changes, and is signed by the person who carried out the calculation to confirm the as-built figures are accurate and the evidence and photographs have been reviewed.
So the builder keeps photographs: the insulation before it is covered, the windows as fitted, the boiler plate. We give you a list of what to collect as the work goes on, and the assessor needs those images at the end. The 2026 edition applies from 24 March 2027, when the Building Regulations etc. (Amendment) (England) Regulations 2026 come into force; until then the 2021 edition governs, and your drawings must name the right one.
What does it cost?
Our fee is from £225, and it is a floor: a new dwelling, a block of flats or a design that needs several runs to pass is quoted in writing. The calculation is not a council charge; building control charges separately for its own checks. If you also need the construction drawings, our building regulations drawings are £545 drawings + £150 survey (£695 total), and an extension that needs steel is sized by structural calculations.
For work that needs no planning permission, the regs and structural package covers drawings and calculations in one fee, and our permitted development checker shows whether your extension needs permission at all. Energy calculations are for building control, not planning, so a permitted project can still need one.
Frequently asked questions
Does my extension need SAP calculations?
Often not. If the new elements meet the limiting U-values and glazing stays within 25% of the extension's floor area, plus any opening it replaces, the elemental route applies. SAP is for schemes that go beyond that, and for new dwellings.
What is the difference between SAP and the Home Energy Model?
Both are approved methodologies for calculating the energy performance of a new dwelling. The 2026 Approved Document L allows either, and its validity periods are set out in the Secretary of State's notice of approval.
How much does a SAP calculation cost?
Our fee is from £225, and it is a floor. We tell you in writing, from your drawings, if your project needs more than one run or more than one dwelling.
Do I need a calculation when I convert a house into flats?
It depends on the work. Send us the proposed drawings and we say in writing, before you pay for a calculation, whether building control is likely to expect one.