The short answer
- Windows, roof windows, rooflights and doors in an extension together shouldn’t exceed 25% of the extension’s floor area, plus the area of any windows and doors the extension removes or covers up.
- A design over the limit can still be approved: a SAP calculation has to show the house with your extension performs no worse than with an extension that follows the rules.
- Every new window, door and rooflight still has to meet its own U-value limit.
1. A worked example
A rear extension of 4 × 5 m has a floor area of 20 m², so 25% is 5 m². It covers the old back door and kitchen window, 3.5 m² together, so the allowance is 8.5 m².
- Sliding doors 4.0 × 2.2 m: 8.8 m²
- Rooflight 1.0 × 2.0 m: 2.0 m²
- Total 10.8 m²: 2.3 m² over the allowance
Measure each window, door and rooflight over its frame. Ask your building control body which dimensions they want if your drawings show both the structural opening and the frame size.
2. U-value limits for each element
| Element | Limit |
|---|---|
| Windows | 1.4 W/m²K, or Window Energy Rating band B |
| Doors more than 60% glazed | 1.4 W/m²K, or Door Energy Rating band C |
| Other doors | 1.4 W/m²K, or Door Energy Rating band B |
| Roof windows and rooflights | 2.2 W/m²K, on the developed area of a rooflight |
These limits apply whether or not the extension is under 25%. Check which edition of Approved Document L applies on the date your application goes in.
3. If your design goes over
- Reduce the glazing, for example a solid panel beside the doors or a smaller rooflight.
- Improve the glazing: lower U-values on the doors and rooflights, for instance triple glazing or a better frame.
- Improve the rest of the fabric: more insulation in the extension’s walls, roof or floor, or in the existing house.
An energy assessor runs the SAP comparison. Building control needs it before the work starts, so do it at design stage, when changing the glazing is still easy.
4. Conservatories and Part O
A conservatory or porch at ground level, under 30 m², separated from the house by walls and doors as good as the outside walls, without the house’s fixed heating extended into it, and with safety glazing, is exempt from Part L, so the rule doesn’t apply. Opening it up to the house later ends the exemption.
Approved Document O, on overheating, applies to new homes, not to extensions. A large glazed extension can still overheat, so solar control glass and a way to ventilate at night are worth considering. For large doors under the same rules, see our design guide to large sliding doors and roof lantern or flat rooflight?
Questions
Does the 25% rule include rooflights?
Yes. Windows, roof windows, rooflights and doors in the extension all count together against the allowance.
What happens if my extension has more than 25% glazing?
It can still be approved if a SAP calculation shows the house with the extension performs no worse than with an extension that meets the rule. That usually means better U-values or more insulation elsewhere.
Do the doors and windows the extension replaces count?
Yes, in your favour. Add the area of any windows and doors that the extension removes or covers up to the 25% allowance.
Does the 25% rule apply to a conservatory?
Not to an exempt conservatory: ground floor, under 30 m², separated from the house by external-quality walls and doors, without the house’s heating extended into it. A glass extension open to the house counts.
Sources
The official documents this guide is based on. Check the current edition before you order.