SAP calculations in London

The three ways an extension can comply

Approved Document L sets out the routes. The first is the elemental route, where every new part of the building meets a limiting U-value. For new elements in existing homes the document states: New elements in existing dwellings, limiting U-values W/(m²·K): roof 0.15, wall 0.18, floor 0.18, windows 1.4 (or WER Band B), doors 1.4.

Glazing has its own cap on that route. The guidance says: Windows, roof windows, rooflights and doors in an extension should not exceed 25% of the extension floor area plus the area of openings removed or covered. A kitchen extension with large sliding doors can pass that cap easily or miss it badly, so we count the openings on the drawings before anyone orders a door.

When the elemental limits do not fit the design, the guidance offers two alternatives: Alternative routes: area-weighted U-value (para 10.9) or a SAP calculation against a notional extension (para 10.10). The SAP route compares your extension against a notional one of the same shape. It gives the designer room to trade, for example a little more glass against a better roof.

We choose the route with you at the drawing stage, because the build-up of walls, roof and floor on the sheets must match whatever the calculation assumes.

New homes and the changing energy rules

A new dwelling is assessed differently from an extension, and the method is changing. For new dwellings AD L 2026 names two methods: the Home Energy Model (HEM) and SAP; validity dates are in the Notice of Approval. If you are planning a new house, a conversion into flats or a large replacement home, your assessor needs to know which edition applies on the day your building notice or full plans are deposited.

The start date is set out in the guidance itself: AD L 2026 takes effect 24 Mar 2027 (24 Sep 2027 for higher-risk building work). Projects already in the system are covered by the transitional rule: AD L/F 2021 continue where plans/notices are deposited before 24 Mar 2027 and work starts before 24 Mar 2028.

In plain terms, the date you deposit plans and the date work starts both matter. We note both on the programme we give you, so the energy edition is never a surprise halfway through the build.

For a new build, our new build drawings and the energy work are planned together, because insulation thickness and window sizes change the wall and roof sections.

Overheating and the energy performance certificate

Energy use is only half the story. Modern, well-insulated homes can run hot, so the Building Regulations also look at summer comfort. The relevant document is plain: Approved Document O (overheating) applies to new residential buildings. For a large glazed extension on a south-facing garden, expect the assessor to ask about shading and opening areas.

A new home also needs an energy performance certificate once built, which is a separate document from the design-stage calculation. The rule reads: An energy performance certificate is required on construction of a new dwelling.

Think of the calculation as the prediction and the certificate as the record. If the two disagree on site, because the builder swapped a window or thinned the insulation, the certificate can come out worse than the design promised. We keep the specification on the drawings for that reason.

How SAP fits with the rest of your drawings

A SAP result is only as good as the information fed into it: floor areas, wall and roof types, window sizes and heating. All of those come from your drawings, so we prepare the data from the same model that produces the plans. That avoids the common problem where the assessor measures from a rough sketch and the figures never match the building.

The calculation sits beside, not instead of, the technical package. Our building regulations drawings show the construction details, and structural calculations cover the beams and foundations. For a full set of working sheets, see our construction drawings.

If you want a second opinion on an existing design before ordering anything, our independent drawing review reads the set against the rules and flags energy risks early.

Cost, timing and who does the calculating

We do not publish a standard price for SAP, because the work ranges from a short check on an extension to a full assessment of a new dwelling. You get a fixed quote after a free constraints check, VAT included, before you commit to anything.

The calculation is carried out by an assessor we coordinate, working from our model. You deal with one point of contact, and the energy figures come back in time for the building regulations submission.

Drawings are normally ready about a week after the survey, and the energy information follows the design freeze rather than holding it up. Send us the address and a short brief to start.

Where we prepare energy information

Every council runs the same national energy rules, but its building control team and local planning conditions shape what is asked for. Our council pages set out the local picture, and you can find yours under architectural drawings by area.

Extending to the rear, side or loft? Start with the matching drawings page, such as rear extension drawings or loft conversion drawings, and ask us to include the energy route in the first design review.