How much glazing can an extension have? The 25% rule explained
In all four UK nations, the energy guidance for house extensions starts from the same rule of thumb: the windows, doors and rooflights in an extension should add up to no more than 25% of the extension's floor area, plus the area of any existing openings the extension covers up or removes.
Going over that figure doesn't mean the design can't be built. It means you need to show, by calculation, that better performance elsewhere makes up for the extra glazing.
The rule in each nation
| Nation | What counts | The allowance | Where |
|---|---|---|---|
| Scotland | Windows, doors and roof lights | 25% of the extension's floor area, plus existing openings built over | Technical Handbook (Domestic), clause 6.2.8 |
| England | Windows, roof windows, rooflights and doors | 25% of the extension's floor area, plus windows and doors that no longer exist or are no longer exposed | Approved Document L Volume 1, paragraph 10.7 |
| Wales | Windows and doors, including roof windows and rooflights (see below) | 25% of the extension's internal floor area, plus windows and doors that no longer exist or are no longer exposed | Approved Document L Volume 1, paragraph 10.11 |
| Northern Ireland | Windows, roof windows, rooflights and doors | 25% of the extension's floor area, plus windows, roof windows, rooflights or doors that no longer exist or are no longer exposed | Technical Booklet F1, paragraph 3.11 |
Working out your allowance
- Take the extension's floor area. It's the floor area of the extension itself, not the whole house. Wales specifies the internal floor area. Scotland's own attic conversion example bases the allowance on all the floor area created (36.4 m²), even though that floor isn't one of the exposed elements in the heat loss calculation (Technical Handbook, Annex 6.A.1).
- Take 25% of it.
- Add the openings you lose. These are existing windows and doors that the extension builds over, or that no longer exist or are no longer exposed because of it: typically the back door or window in the wall the extension is built against.
- Compare with your design. Add up every window, door and rooflight in the extension. If the total is within the allowance, you meet the glazing limit.
For example, a single-storey rear extension with a 20.0 m² floor, built against a wall with a 1.9 m² back door that will be removed:
- 25% of 20.0 m² = 5.0 m²
- plus the 1.9 m² door lost = 6.9 m² allowance
A 3.6 m² sliding door, a 1.2 m² window and a 1.0 m² rooflight add up to 5.8 m², so that design is within the limit.
What counts towards the total
- Whole units, not just the glass. England and Wales both treat a window or door as the whole unit, including its frame (Approved Document L Volume 1, paragraph 10.6; the Welsh Approved Document L Volume 1, paragraph 10.24). So measure the unit, not the visible glass.
- Doors count, including solid ones. The limit covers doors as well as glazing.
- Rooflights and roof windows. Scotland, England and Northern Ireland name them explicitly. Wales's paragraph 10.11 says only "windows and doors", but the Welsh document's own Table 10.1, headed "Windows and doors", lists roof windows and rooflights under that heading. So count them in Wales too. Otherwise an extension could have unlimited rooflights without any calculation.
Is there a minimum?
Too little glazing has its own problems. Wales advises against going below 20% of the extension's internal floor area, because the extension, and the part of the house it adjoins, are likely to have low levels of daylight and rely more on electric lighting (paragraph 10.12). Northern Ireland gives the same caution as a general guide (paragraph 3.11).
If you want more glazing
You have two main options.
A compensatory calculation. You show that the extension as a whole loses no more heat than a version of itself that meets the 25% limit, usually by improving the insulation of the walls, floor or roof, or by using better-performing glazing. In Scotland this is the compensatory approach, and the handbook says the 25% "may be exceeded" where it's used (clause 6.2.8). England, Wales and Northern Ireland use an area-weighted U-value comparison instead. See What is a compensatory heat loss calculation, and when do you need one? and What is an area-weighted U-value?
A whole-dwelling assessment. Every nation also allows a route based on SAP (the Standard Assessment Procedure) that assesses the whole house, extension included. It is most practical where SAP data for the existing house is already available.
Northern Ireland adds that larger areas of glazing "may be acceptable, especially where this is required to make the extension consistent with the external appearance or character of the building being extended", with compensating measures (paragraph 3.12).
Not to be confused with
In England, enlarging a window or adding a new one in the existing house has its own 25% rule, based on the whole dwelling's floor area rather than an extension's. If the dwelling's windows, roof windows, rooflights and doors would exceed 25% of its total floor area, compensating measures should be taken to improve its energy efficiency (Approved Document L Volume 1, paragraph 10.5).
Sources
This guide covers extensions to homes. Extensions to other buildings follow different rules. It refers to the editions in force in September 2026:
- Scotland: Building Standards Technical Handbook, Domestic (April 2026), clause 6.2.8 and Annex 6.A.
- England: Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments), paragraphs 10.5 to 10.7. A 2026 edition applies from 24 March 2027.
- Wales: Approved Document L, Volume 1: Dwellings (2022 edition), paragraphs 10.11, 10.12 and 10.24. A 2026 edition applies from 4 March 2027.
- Northern Ireland: Technical Booklet F1 (2022), paragraphs 3.11 and 3.12.