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Part L for Extensions & Conversions

Working to Part L 2021 (Approved Document L, volumes 1 and 2)

Work to existing buildings is covered by Part L, but most of it does not need a full new-build calculation. Extensions and conversions usually comply element by element, meeting the limiting U-values and services standards in Approved Document L. A calculation is needed when the design departs from those standards, most often through extra glazing, or when the work is large enough to be treated as a new building.

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Which Route Does Your Project Need?

DwellingsDomestic extensionsExtensions to houses and flats with more glazing than the standard allowance, shown to comply with a SAP comparison.Domestic extensions DwellingsConversions to dwellingsBarns, offices and other buildings converted to homes, and dwellings created by dividing a building.Conversions Non-domesticNon-domestic extensionsExtensions to offices, schools, industrial and other non-domestic buildings, from element by element to a full BRUKL.Non-domestic extensions

Domestic Extensions

An extension that meets the limiting U-values, with a glazed area no greater than 25% of the extension floor area plus the area of any windows or doors removed by the work, complies without a calculation. Above that, or where the fabric falls short of the standards, a SAP calculation compares the existing dwelling with the proposed extension against the same dwelling with a notional extension built to the standards. The proposal complies if its emissions and primary energy are no worse. Better fabric elsewhere, or improvements to the existing house, can offset the extra glazing.

Conversions

Converting a building to a dwelling, or changing the number of dwellings in a building, brings the work under Part L. Elements being renovated or replaced need to meet the standards for existing buildings, and a new dwelling needs an EPC, which is produced from a SAP assessment. Where the existing fabric cannot reasonably reach the standards, for example on a listed or traditional building, SAP can be used to show the overall performance of the dwelling.

Non-Domestic Extensions

An extension to a non-domestic building that is over 100m² and over 25% of the existing building's floor area is treated as a new building, and needs a BRUKL report from SBEM or dynamic simulation. Smaller extensions meet the fabric and services standards element by element. In existing non-domestic buildings over 1000m², an extension or an increase in installed services capacity can also trigger consequential improvements to the existing building.

Not sure which applies? Get in touch with the drawings and we will confirm the route before any modelling starts.

Last reviewed: November 2026

Questions

FAQs

Do I need SAP calculations for a single-storey house extension?

Only if the design does not meet the standard requirements, usually because the glazed area is above the 25% allowance or the fabric does not meet the limiting U-values. Otherwise building control can sign it off element by element.

Does a conversion need an EPC?

Yes. A new dwelling created by a conversion needs an EPC on completion, produced from a SAP assessment of the finished dwelling.

What are consequential improvements?

Improvements to the existing building required when extending or adding services to a non-domestic building over 1000m², such as upgrading lighting, controls or fabric, where they are technically, functionally and economically feasible.

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Send us the drawings and we will confirm whether a calculation is needed.

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