Choose the correct set of standards
Approved Document L distinguishes new dwellings from new or replacement elements in existing dwellings, and from retained or renovated elements. Do not copy a new-build limiting value into an extension specification without checking which table and assessment route applies. [2]
Under the current 2021 edition incorporating 2023 amendments, Table 4.2 gives maximum U-values for new fabric in existing dwellings including 0.15 W/m²K for roofs, 0.18 for walls and 0.18 for floors. The table contains qualifications and alternative rating routes for some fittings; it is not a stand-alone specification of insulation thickness. [2]
Ask for the proposed build-up and calculated performance. Different materials and junctions need coordinated detailing, and a product brochure should not be treated as the U-value of the whole finished wall or roof. [2]
How the extension glazing allowance works
The straightforward extension route limits the total area of windows, roof windows, rooflights and doors to 25% of the extension's floor area, plus the area of existing windows and doors that disappear or are no longer exposed because of the extension. This is not simply a ban on glazing more than a quarter of the new wall. [2]
For illustration, a 20 m² extension starts with 5 m² under the 25% calculation, before adding any qualifying existing openings. This is an arithmetic example, not a project-specific approval.
More glazing can be assessed through alternative approaches, including an area-weighted comparison or a whole-dwelling SAP comparison described in the guidance. Resolve the assessment before committing to large rooflights or glazed doors. [2]
SAP and the wider design
SAP is used in relevant dwelling energy assessments; it is not simply a label applied to any insulation note. The assessment needs the actual geometry, fabric and services information, with revisions coordinated when the design changes. New-home compliance also involves performance targets beyond individual limiting U-values. [2]
Ventilation and overheating need separate attention. Meeting a heat-loss calculation does not demonstrate adequate fresh air or acceptable summer temperatures. [3] [4]
What changes from 24 March 2027?
The Future Homes and Buildings Standards introduce new provisions from that date, subject to transition and separate higher-risk provisions. The Future Homes Standard concerns new homes, while the wider amendments also affect relevant work and services in existing buildings. It is not a universal retrofit requirement. [5]
Use the published 2026 Part L guidance only with the correct commencement and transitional position for the project. Record that basis on the technical specification and retain the final calculations for completion. [6] [5]