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Guide
Part L 2026 Window U-Values: The Actual-Size Rule
Approved Document L, Volume 1 (2026 edition) changes how window and door U-values are determined for new dwellings in England. From 24 March 2027 for work outside the higher-risk-building regime, the standard reference-size calculation route closes: use an appropriate hot-box measurement or calculate the actual size and full configuration. This guide separates that method change from the whole-dwelling compliance target and the limiting backstop.
Trade onlyWe fabricate and supply the trade. Homeowners are routed to vetted installers.
Primary-source verifiedFigures below were checked against Approved Document L Volume 1, 2026 edition (gov.uk), not summaries.
Aluminium specialistsDecorative aluminium elements have the largest effect under the new methodology.
What actually changes on 24 March 2027?
Under the 2026 edition, the standard reference-size calculation route closes for new dwellings in England. Where the calculation route is used, it must use the actual size and configuration and include the frame sections, sills or thresholds, frame extenders, dummy sashes, dummy transoms or mullions, decorative bars within the glazing cavity and non-load-bearing couplers where present.
Approved Document L permits two determination routes for new-dwelling windows and doors: an appropriate hot-box measurement under BS EN ISO 12567, or calculation at actual size and configuration under BS EN ISO 10077-1 and 10077-2. The approved dwelling methodology and building-control process determine compliance; product evidence must match the unit and configuration that are specified. Read the official 2026 Approved Document L publication.
Request current product evidence for the selected route, size, glazing, hardware and project before relying on the specification.
The figures: limiting versus notional
Two concepts circulate and confusing them costs jobs. The limiting values in Approved Document L Volume 1 Table 3.1 are backstops: windows 1.6 W/m2K and doors 1.6 W/m2K for new dwellings. They are not, by themselves, a promise that the dwelling will pass its whole-building energy calculations.
For EXISTING dwellings (replacement work), the limit is 1.4 W/m2K or a Window Energy Rating of Band B. Replacement-window calculations may still use the standard size range (1.23 m +/-25 percent wide by 1.48 m +/-25 percent high) provided the ACTUAL configuration is used.
The actual dwelling is compared with a notional dwelling using the approved methodology, and the design can vary from the notional specification only if the required whole-dwelling targets and limiting standards are met. Treat 1.6 W/m2K as the limiting backstop, not as the project target; obtain the design-stage target and confirm the selected units against it.
Why size and configuration change the result
The calculation route is not a range-wide declaration. Approved Document L paragraph 3.2 requires the actual opening size and configuration, including the relevant frame sections, thresholds, extenders, dummy sashes, transoms or mullions, glazing-cavity decorative bars and non-load-bearing couplers.
Changing those details can therefore change the whole-unit result even when the base system and glazing are unchanged. Give the energy assessor evidence for the opening that is actually specified, not a result for a cleaner or differently sized reference unit.
Small windows are the hidden risk
A smaller window carries proportionally more frame to glass, and frames perform worse than glass. A configuration that passes at 1200 x 1400 mm can fail at 800 x 900 mm. Bathrooms, cloakrooms and multi-light designs are where specs quietly break.
Where the calculation route is used, each opening’s actual size and configuration must be modelled. An appropriate hot-box measurement remains the alternative route in paragraph 3.2.
What suppliers must do now
For new-dwelling work entering the 2026 standard, replace generic range figures with configuration-specific evidence early enough for the designer and energy assessor to use it.
- Record whether the quoted U-value is supported by a BS EN ISO 12567 hot-box measurement or a BS EN ISO 10077-1 and -2 calculation at actual size and configuration.
- When using the calculation route, do not reuse a standard reference-size result across different sizes or configurations.
- Re-check any job currently in design and give the dwelling's assessor evidence that matches every selected opening.
- For decorated aluminium specifications, budget for triple glazing or plan the SAP trade-off early.
How VitrineAlu supports Part L evidence
We fabricate and supply aluminium to the trade. For new-dwelling work, we record the quoted configuration and provide the applicable evidence route: hot-box measurement where it applies, or an actual-size/configuration calculation. That gives the project team the correct U-value input. Final compliance remains a whole-dwelling and building-control decision.
Sending us your schedule early is the cheapest insurance in this transition. Send us the openings and we will quote the system and flag anything the new methodology puts at risk.
Common questions
Does Approved Document L 2026 require every new-dwelling window to be calculated?
No. It permits either an appropriate hot-box measurement under BS EN ISO 12567 or a calculation at the actual size and configuration under BS EN ISO 10077-1 and 10077-2. What closes is the standard reference-size calculation route for new dwellings.
When does the 2026 actual-configuration rule take effect in England?
The 2026 edition takes effect on 24 March 2027 for building work that is not connected with higher-risk building work, subject to the published transitional arrangements. Higher-risk-building work has a later 24 September 2027 date. Confirm which regulatory standard applies to the project before relying on a value.
Is 1.6 W/m2K the target U-value for every new-dwelling window?
No. Table 3.1 gives 1.6 W/m2K as the limiting backstop for windows and doors in new dwellings. The dwelling must also pass the target primary-energy, emissions and fabric-efficiency calculations, so the project may need better performance. Confirm the project target and the selected unit at quote.
Can a standard reference-size calculation still be used for replacement windows?
Approved Document L keeps a bounded standard-size calculation route for replacement windows in existing dwellings, provided the actual configuration is used. That exception does not restore the reference-size calculation route for windows in new dwellings.
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