What is the 25% glazing rule for extensions?

Part L of the Building Regulations limits glazing area in extensions to 25% of the extension floor area. This includes all windows, glazed doors and roof glazing. If you exceed 25%, you must compensate by improving insulation performance elsewhere or use better-performing glazing. A SAP or SBEM calculation is needed to demonstrate compliance.

Can I have more than 25% glazing in an extension?

Yes, but you must compensate. Common strategies include using triple glazing (U-value ~0.8 instead of 1.4), improving wall insulation beyond minimum requirements, or using a better-performing floor/roof. The whole extension must perform at least as well as one built to standard specifications — this is demonstrated through SAP/SBEM calculations.

What U-value do extension windows need?

Part L 2021 requires a maximum U-value of 1.4 W/m²K for windows and 1.2 W/m²K for roof windows/skylights. The target U-value is 1.2 W/m²K for the notional building comparison. Most double glazed units achieve around 1.2–1.4, while triple glazed units achieve 0.7–0.9 W/m²K.

Does glazing in doors count towards the 25% limit?

Yes. The glazed portion of doors (not the frame or solid panels) counts as glazing for the 25% calculation. Fully glazed bi-fold or sliding doors contribute their full opening area. Only the actual glazed area counts — solid panels at the bottom of doors are excluded.

Do roof windows and skylights count towards the 25% limit?

Yes, roof glazing including Velux windows, sun tunnels and sky lanterns all count towards the total glazing area. Roof glazing also has a stricter U-value limit of 1.2 W/m²K compared to 1.4 for vertical windows, because heat loss through horizontal glazing is higher.

TradeCalculator

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

Check whether your extension glazing area complies with Part L requirements (max 25% of floor area) and calculate U-value compensation options.

Extension floor area

Number of windows

Average window width

Average window height

Glazed doors

Glazing type

How We Calculate This

This calculator totals all glazing area in your extension and checks it against the Part L 25% rule.

The 25% rule

Total glazing (windows + glazed doors + roof glazing) must not exceed 25% of the extension floor area. If it does, compensation through improved U-values or enhanced fabric performance is required, demonstrated via SAP/SBEM calculations.

U-value targets

Windows: 1.4 W/m²K max (1.2 target). Roof glazing: 1.2 max. Better U-values allow larger glazing areas through compensation.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.