Planning bifold doors, a large window or a couple of rooflights? You may have heard that glazing cannot exceed 25% of your extension’s floor area. That is only part of the calculation.
For extensions in England, the 25% glazing rule also allows for existing windows and doors covered or removed because of the extension. You could have more room for glass than you think.
How does the 25% rule work?
Start with 25% of your extension’s internal floor area. Then add the area of existing windows and doors that will no longer face outside because of the extension.
Opening allowance = (extension floor area × 0.25) + qualifying existing openings
This is the reference method opening allowance in Approved Document L, paragraph 10.7(d). It is an opening area check: the insulation and building services need to meet their requirements too.
Calculating your extension’s glazing limit
Imagine an extension measuring 6m x 4m. It covers an existing window measuring 2m x 1m and a door measuring 1m x 2m.
| Calculation | Area |
|---|---|
| Extension floor area: 6m x 4m | 24 m² |
| 25% of the floor area: 24 × 0.25 | 6 m² |
| Existing window: 2m x 1m | 2 m² |
| Existing door: 1m x 2m | 2 m² |
| Total opening allowance | 10 m² |
Now add up the proposed openings. Bifolds measuring 3m x 2.1m use 6.3 m². A 1.2m x 1m window adds 1.2 m², and two 1m x 1m rooflights add another 2 m².
That gives 9.5 m² of proposed openings against a 10 m² allowance. The design passes this area check with 0.5 m² to spare. Without including the covered window and door, the allowance would be just 6 m² and the same design would appear to exceed it by 3.5 m².
What should you include in the measurements?
Count the extension’s windows, external doors, roof windows and rooflights. Measure the complete opening, including the frame, rather than just the visible glass. An opaque external door counts too, despite the shorthand name “glazing allowance”.
For the covered openings, count the area of all openings that are being covered by the new extension, even if the wall they were on is being knocked through.
Can you exceed the 25% glazing rule?
Yes. Exceeding the opening allowance does not mean you have to shrink the bifolds or remove a rooflight.
An Area Weighted U-value calculation can check whether better insulation or glazing balances the extra heat loss. Use our free extension heat loss calculator to try your measurements and calculated U-values. If that is not enough, a whole house SAP comparison can consider improvements to the existing home. Our extension calculations guide explains how the options fit together.
Use our free extension glazing calculator to check your own measurements. If your design exceeds the allowance, we can help you work out the next step.
