How much glass can an extension have?
In England, the windows, roof windows and doors in an extension should not normally exceed 25% of the extension's floor area plus the area of any windows and doors the extension removes or covers. More glazing is allowed if calculations show the whole house still meets the energy standard, which is how most glass-heavy extensions are approved.
A worked example
A 20 m² rear extension that covers an existing 4 m² patio door can normally have up to 5 m² (25% of 20 m²) plus 4 m², so 9 m² of glazing without further calculations. A three-panel sliding door of 3.6 m × 2.1 m is already 7.6 m², which leaves little for rooflights.
Going beyond the limit
To exceed the allowance, the designer shows by calculation that the whole house, with the extension, performs at least as well as it would with a compliant extension. Upgrading insulation elsewhere, using better glass, or improving the heating can all help.
Overheating
Generous glazing on a south or west-facing extension can make it uncomfortably warm in summer. Solar-control glass, opening vents, roof overhangs and blinds all help, and we design them in rather than adding them later.
Other UK nations
Wales, Scotland and Northern Ireland apply their own energy standards to extensions, with similar principles. We check the rules for your address.
This guide is general information, not legal or professional advice for your property. Rules can change; we check the current position for your address at the survey.