Glossary
SAP calculations and proving Part L compliance in London
Standard Assessment Procedure — the government's energy-efficiency calculation for dwellings.
Standard Assessment Procedure — the government's energy-efficiency calculation for dwellings.
SAP — the Standard Assessment Procedure — is the government's methodology for assessing the energy performance of dwellings. A SAP calculation is required for every new dwelling, including those created by conversion, and produces both the compliance check against Part L and the EPC.
Usually not. An extension normally complies through the elemental route — meeting a limiting U-value for each new thermal element. A SAP calculation is required for a new dwelling, a conversion that creates one, and for an extension that relies on the whole-dwelling calculation route to justify extra glazing.
| Element | Limiting U-value, new thermal element | Route |
|---|---|---|
| External wall | 0.18 W/m²K | Elemental |
| Pitched or flat roof | 0.15 W/m²K | Elemental |
| Ground floor | 0.18 W/m²K | Elemental |
| Windows, doors and rooflights | 1.4 W/m²K | Elemental |
| Glazing area in an extension | 25% of the new floor area, plus openings closed up | Elemental |
| New dwelling or conversion | whole-dwelling calculation | SAP |
Source: Approved Document L, Volume 1: Dwellings (2021 edition, incorporating the 2023 amendments), read at 1 September 2026 — the limiting values for new thermal elements in an existing dwelling and the glazing allowance for extensions. SAP is the Standard Assessment Procedure published by the Department for Energy Security and Net Zero; exceeding the glazing allowance is the commonest reason an extension is pushed onto the calculation route.
For any new dwelling, and for a dwelling created by material change of use — converting a house into flats, or an outbuilding into a self-contained unit. Extensions to existing dwellings normally use the elemental U-value route instead and need no SAP at all.
The exception is a large glazed extension. Where the glazing exceeds 25% of the extension's floor area plus displaced openings, compliance is shown by calculation, and that calculation is SAP-based.
Design stage SAP is produced from the drawings and specification before construction, and demonstrates compliance for the building control submission. As-built SAP is updated with what was actually installed and with the air test result, and generates the EPC lodged on the register.
Gaps between the two are the common problem: a specified triple-glazed unit substituted for double, or a heat pump swapped for a boiler, and the as-built SAP fails.
Fabric U-values, thermal bridging, air permeability, the heating system efficiency and controls, hot water storage, ventilation type, and any renewable generation. Thermal bridging is the most commonly underestimated input — using default psi values rather than accredited construction details can cost several SAP points.
How sap calculation plays out in Hammersmith and Fulham: London Borough of Hammersmith and Fulham (Hammersmith & Fulham Council) maintains 45 conservation areas, the Hammersmith Grove Conservation Area and the Brackenbury Village Conservation Area among them, and an Article 4 direction is in force there, so permitted development cannot be assumed. Anyone dealing with sap calculation at a Hammersmith and Fulham address is therefore working to a £548 householder application decided in 8 weeks, or a £249 prior approval over 6 weeks where sap calculation still qualifies.
| Required for | New dwellings and dwellings created by conversion |
|---|---|
| Not required for | Most extensions |
| Stages | Design stage and as-built |
| Produces | Part L compliance and the EPC |
| Existing homes use | RdSAP, a reduced-data version |
| Key inputs | U-values, thermal bridging, airtightness, heating |
Usually not — the elemental U-value route applies. A heavily glazed extension may need one.
The reduced-data version used to produce EPCs for existing homes.
An accredited on-construction domestic energy assessor.
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