Part L · England · Free to use
Extension glazing allowance calculator (25% rule)
The 25% rule: how much window, rooflight and glazed door area a home extension can have.
Approved Document L Volume 1 (2021, including 2023 amendments)
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Windows and doors in the existing wall that the extension removes or encloses.
Part L · Result
0.6 m² over the allowance
Allowance 8.4 m², proposed 9 m².
Check failed- 25% of extension floor area: Calculated result0.25 × 24 m²6 m²
- Plus openings no longer exposed: Calculated result2.4 m²
- Total openings proposed: Check failedmax 8.4 m²9 m²
- Openings as % of floor area: Calculated result37.5%
- Over the limit: use the area-weighted U-value route (para 10.9) or show the whole dwelling complies with SAP.
Clauses this relies on
Source edition and scope
Approved Document L Volume 1 (2021, including 2023 amendments).
Uses the 2021 edition including 2023 amendments, paragraph 10.7(d). The 2026 standards take effect from 24 March 2027, or 24 September 2027 for higher-risk building work, subject to transitional arrangements.
This is a preliminary check of the conditions shown. Confirm the edition, project circumstances and any additional requirements with your architect or building control.
A design aid, not legal advice. Checked against the Building Regulations 2010 for England; confirm with building control.