The 2026 update to Part L sets significantly tougher performance standards for windows and doors in new homes, tightening thermal targets and installation rules that will reshape fenestration specification on UK housing sites.
Stricter U-values in new dwellings – For new dwellings, windows and doors must now meet a maximum U-value of 1.6 W/m²K on an area‑weighted basis, bringing all openings into line with the same headline limiting value. This 1.6 W/m²K cap applies to windows, roof windows and curtain walling, and separately to doors including glazed doors, with dormer window roofs and cheeks explicitly treated as part of the main roof and wall fabric. The document stresses that to hit the overall fabric energy efficiency target, many elements – including glazing – will in practice have to outperform these limiting values.
How window and door performance must be calculated – Part L 2026 tightens how U-values are derived, insisting on whole‑product performance for windows and doors in new dwellings. U-values must be measured using the hot-box method or calculated to BS EN ISO 10077-1 and 10077-2, and must include frame sections, sills or thresholds, add-ons, dummy sashes and mullions, decorative glazing bars and non‑structural couplers where present. Only dead‑load‑bearing items such as bay poles can be excluded, and U-values for windows and roof windows must be based on a vertical position, with separate rules for horizontal rooflights
Installation, airtightness and thermal bridging around openings – The guidance puts new emphasis on how windows and doors are installed to maintain the insulated plane and airtightness in new homes. Window and door units must be located so that the inner face overlaps the inner face of the external leaf – 30–50 mm for windows and 50 mm for doors – so the frame is contiguous with the wall insulation layer. Tolerances around openings should be minimal and in line with BS 8213‑4, with fully insulated cavity closers, perimeter insulation at thresholds and taped junctions to the primary air barrier to reduce both thermal bridging and air leakage.
Limits on total window and door area in new dwellings – When new windows or doors are created in existing homes or when a dwelling is extended, the total area of windows, roof windows, rooflights and doors is generally capped at 25% of the total floor area of the dwelling or extension, unless compensatory energy efficiency measures are taken. In change‑of‑use and conversion scenarios that create new dwellings, openings in the newly created home are also expected not to exceed 25% of floor area, with any additional area requiring compliance to be demonstrated via Standard Assessment Procedure calculations. The document clarifies that these area limits are intended to control heat losses and gains through glazing and doors as part of meeting the primary energy and emission targets for new homes.
Existing stock and character buildings feeding into new supply – Although focused on new performance, the Approved Document sets out how replacement windows and fully glazed external pedestrian doors in existing dwellings must now achieve either 1.4 W/m²K or a minimum Window or Doorset Energy Rating, unless this would compromise the character of protected buildings. In those cases, single glazing should be paired with low‑emissivity secondary glazing or centre‑pane U-values of 1.2 W/m²K, signalling that even heritage‑driven exceptions are expected to move towards higher thermal performance. For housebuilders working on regeneration and conversion projects that create new dwellings, these strengthened back‑stop values for replacement units set the baseline for fenestration packages that feed into the new‑build supply pipeline.
Why This Matters: The real shift is not a sudden tightening of replacement-window U-values. It is that, in new-build homes, compliance is moving toward more realistic product and junction assessment: actual-size whole-unit window and door calculations, tougher treatment of thermal bridges in HEM, and no special curtain walling dispensation. That is likely to favour suppliers and installers who can provide strong whole-window data, robust junction details and cleaner evidence for as-built compliance.
