There is no absolute 25% ban on glazing in an extension. Approved Document L's standard approach limits the combined area of windows, roof windows, rooflights and doors to 25% of the extension floor area plus qualifying openings lost from the existing house, but alternative calculated compliance routes are available. Large bi-fold doors and rooflights should therefore be assessed together, not as separate allowances. [1]
Understand what the calculation includes
For an extension, the standard approach in the currently applicable 2021 edition of Approved Document L, incorporating 2023 amendments, allows the sum of:
- 25% of the extension's floor area; and
- the area of existing windows and doors that disappear or are no longer exposed because of the extension.
The calculation includes doors as well as windows and rooflights. Ask for an opening schedule with the dimensions used, rather than estimating the glass area from an elevation. [1]
As an illustration only, a 20m² extension gives 5m² under the 25% part of the formula. If 3m² of qualifying existing openings are lost, the total allowance under that approach would be 8m². The actual project's areas and qualifying openings must be measured; the example is not a design approval.
What happens above that amount?
Approved Document L provides an area-weighted U-value comparison as an alternative. It also allows a Standard Assessment Procedure, or SAP, comparison of the existing dwelling plus the proposed extension against the dwelling plus a notional compliant extension. The relevant energy and emissions performance measures must be demonstrated. [1]
This is a calculation exercise, not permission to add glass on the understanding that another part of the house will somehow be “more efficient”. Obtain the assessment before ordering the glazing, and make sure the specified insulation and products match the model.
Coordinate light with structure and comfort
Rooflights require structural consideration where roof members are interrupted. Large door openings also need the surrounding structure designed; their width is not just a glazing supplier's decision. [2] [3]
Ask the designer to compare rooflight location, opening arrangements and the glazed wall as a complete room. Ventilation must be designed for the altered accommodation, rather than relying only on the ability to open a large garden door. [4]
Near a boundary, rooflights and external openings also raise fire-safety considerations. A product's thermal performance does not establish that its location and construction satisfy fire requirements. [2] [5]
What changes in March 2027?
The published 2026 Approved Document L retains the extension-specific 25%-plus-lost-openings approach and alternative methods. Its wider specification and applicable edition must nevertheless be checked for the project. [6]
For ordinary non-higher-risk work, the changes take effect on 24 March 2027, with transitional protection dependent on the relevant notice or application and commencement requirements. A planning application date alone does not secure that protection. [7]
Keep the final glazing schedule, energy assessment and construction drawings consistent. A late change to larger doors or additional rooflights should be checked before it is accepted.