Extension Glazing Calculator — Part L Area Limits & U-Values

Approved Document L Volume 1 (2021) para 10.7.d limits the total area of windows, roof windows, rooflights and doors in an extension to the sum of (i) 25% of the extension floor area, plus (ii) the area of any existing windows and doors that no longer exist or are no longer exposed because of the extension. So if the new extension encloses an old window or door, you can add that area to the 25% allowance. If you still exceed the allowance, you do not automatically fail — you demonstrate compliance another way (see below).

Yes. AD L gives two alternative routes. Para 10.9: the area-weighted U-value of all the extension's thermal elements must not exceed the area-weighted U-value of a notional extension of the same size and shape that just meets the 25% rule — so better glazing (e.g. triple ~0.8 instead of 1.4) and/or better walls, floor and roof can compensate for extra glass. Para 10.10: a full SAP calculation showing the dwelling-plus-extension does not exceed the targets for the dwelling plus a compliant notional extension. A glazing U-value on its own does not earn extra opening area — a calculation is required.

Under AD L Volume 1 (2021) Table 4.2 the limiting (worst-acceptable) U-value for a new window or glazed door in an extension is 1.4 W/m²K, or a Window Energy Rating of Band B minimum. This applies to all materials, including timber: the old 1.6 W/m²K / WER Band C concession for timber windows was a transitional provision that expired on 14 June 2023, so since 15 June 2023 timber windows must also meet 1.4 W/m²K / WER Band B. Rooflights are allowed up to 2.2 W/m²K. Separately, the 'notional dwelling' specification used only in the SAP target comparison assumes windows and glazed doors at 1.2 W/m²K — that is a comparison benchmark, not a value every window must meet. Most modern A-rated double-glazed units achieve about 1.2 W/m²K and triple-glazed units about 0.7–0.9.

Yes. The glazed portion of doors counts towards the opening-area total alongside windows and rooflights. Fully glazed bi-fold, sliding or French doors contribute their full opening area. Where a door is only part-glazed, count the glazed portion. (Note AD L counts the door opening area for the 25% allowance; the U-value limit applies to the whole door unit.)

Yes, roof glazing — roof windows, rooflights, sun tunnels and lantern/sky-lantern glazing — all count towards the opening-area total. Contrary to a common misconception, rooflights are allowed a HIGHER (less stringent) U-value than vertical windows: AD L Table 4.2 sets the rooflight limit at 2.2 W/m²K versus 1.4 W/m²K for windows. Rooflight U-values are measured on the developed surface area of the unit (Ud-value), per BRE BR 443 and NARM NTD02.