SAP Calculations

What a SAP calculation is

SAP — the Standard Assessment Procedure — is the government’s method for calculating the energy performance of a dwelling, and the calculation Building Control uses to check that a new home complies with Part L. Your dwelling’s floor areas, volumes, U-values, junction details, air tightness, heating, hot water, ventilation, lighting and renewables are modelled and compared with a notional dwelling — the same size and shape as yours, built to a reference specification. Beat it on every count and the dwelling complies.

Part L 2021 work is assessed in SAP 10.2. SAP 10.3, approved on 24 March 2026, is the methodology for the Future Homes Standard (schemes notified from 24 March 2027) and carries different fuel factors — what changed. The Home Energy Model, which will eventually take over, is not yet approved.

The four tests a new dwelling has to pass

It is easy to think of SAP as “the carbon test”. Under the 2021 edition of Approved Document L a new dwelling has to clear four hurdles at once, and they are checked independently:

Miss one and the dwelling fails. The limiting values are generous — a wall at 0.26 W/m²K passes the backstop — but the notional dwelling is not, so a design that merely clears the backstops will usually fail the targets.

What you are measured against

The notional dwelling in Approved Document L is more demanding than people expect, and knowing what is in it explains most SAP results. It has walls at 0.18, floors at 0.13, a roof at 0.11 and windows at 1.2 W/m²K; an air permeability of 5 m³/(h·m²); a gas boiler at 89.5% with a 55°C flow temperature; waste water heat recovery on every shower; 80 lumens-per-watt lighting; and solar PV sized at 40% of the ground floor area divided by 6.5 — so a 60 m² footprint carries about 3.7 kWp of panels before you have started. That is why fabric improvements alone rarely get a modern dwelling through without PV or a heat pump, and why the cheapest route to a pass is usually a combination rather than one heroic measure. Calculated junction Ψ-values are often the cheapest of all — our free ΨMonkey calculator shows you the number before you commit.

When you need one

• New dwellings — every house, and every flat in a block, at design stage and as built, with the EPC on completion.
• Conversions — barn conversions, office-to-residential, a house split into flats. Part L applies in full, but through a different route: there is no carbon target, compliance is shown element by element, and a SAP is the usual way to demonstrate it.
• Extensions — only when they are over-glazed. Keep the windows, doors and rooflights within 25% of the extension’s floor area and the extension complies on its element U-values; go over it and a whole-dwelling SAP is one of the two ways through.

How a new-build SAP runs, start to finish

Before work starts. We model the dwelling from your drawings and specification and produce the design-stage calculation. The regulations require the target rates, the as-designed rates and the list of specifications the calculation depends on to reach Building Control by the day before work begins, reported on the design-stage BREL report. This is the version to get right: if it fails, everything is still a drawing.

During the build. The specification list is now the contract. Substitutions on site — a different insulation, cheaper glazing, a change of boiler — all have to be reported at the end and can turn a pass into a fail. Since the 2021 edition the builder also has to photograph the construction at six stages — foundations and ground floor, each wall type, each roof type, each opening type and its reveals, the airtightness details, and the services and pipework insulation — showing the insulation continuous and properly fitted. Date-stamped and located. Tell the site team on day one; retrofitting photographs is not possible.

At completion. The air test is done and the measured result goes into the as-built SAP, along with the heating actually installed, the lighting and every change. The completion-stage BREL is signed by the assessor and the builder and, with the photographs, notified to Building Control within five days of completion. The EPC is lodged from the as-built SAP by an accredited on-construction assessor, given to the owner and notified to the local authority in the same five-day window, and the home user guide goes to the occupier. Building Control cannot issue the completion certificate without the EPC.

Where we work

Everywhere in England and Wales, from the desk. A SAP calculation is a desktop assessment from your drawings and specification — there is no site visit in it — so where the site is makes no difference to how we do it or how quickly. We assess plots across England and Wales — London boroughs, the Midlands, the North, the South West — and the only local question that ever matters is which planning authority you are under, because a few of them ask for more than Part L.

London is the one that does it systematically: the London Plan’s 35% carbon target, borough energy statements and carbon offset payments sit on top of Part L for many schemes there, so it has its own page. Wales has its own Approved Document L (2022 edition) with its own, tighter, carbon target; the process is the same.

Why Energycount?

We assess hundreds of dwellings a year across the UK, advising from initial design through to completion — not just running the numbers, but helping specify the most cost-effective route to a pass. Thermal bridging is often the cheapest win: our ΨMonkey psi calculator lets us (and you) calculate accurate junction Ψ-values instead of using punitive defaults.

SAP is rarely the only report a dwelling needs — see the full stack of compliance services for residential buildings if you want the lot priced together, or the Compliance Pack for SAP, EPC, water, air and sound as one job.

Need it fast? Ask about ASAP, our priority fast-track SAP service.

What is changing in 2027

Approved Document L 2026 — the Future Homes Standard — takes effect on 24 March 2027 (24 September 2027 for higher-risk buildings). Projects with a building notice or full plans submitted before that date can still build to the 2021 targets if work starts by 24 March 2028. The notional dwelling gets a heat pump and a tighter air permeability, and a new requirement L3 makes on-site renewable generation a legal requirement rather than something the notional dwelling merely assumed. The limiting U-values do not change. If your scheme straddles the date, tell us and we will tell you which side of it you fall.