What building regulations apply to a house extension?
Almost every house extension needs building regulations approval, whether or not it needs planning permission. The regulations cover structure, fire safety, insulation, ventilation, drainage and safe glazing. In England new extension walls must reach a U-value of 0.18 W/m²K, roofs 0.15 and floors 0.18, and an inspector signs the work off with a completion certificate.
Full plans or building notice?
In England and Wales you can submit full plans, which are checked before work starts, or a building notice, where the work is inspected as it goes. For extensions we usually recommend full plans: problems are found on paper rather than after the steel is in. In Scotland a building warrant is required before work starts on almost any extension.
Insulation standards for extensions in England
| Element | Limiting U-value (W/m²K) |
|---|---|
| Walls | 0.18 |
| Roofs | 0.15 |
| Floors | 0.18 |
| Windows | 1.4 (or energy rating band B) |
| Rooflights | 2.2 |
| Doors | 1.4 |
These figures come from Approved Document L (2021 edition). Wales, Scotland and Northern Ireland set their own standards, which are broadly similar.
How much glass can an extension have?
In England, 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 or doors that the extension removes or covers. More glass is possible if calculations show the whole house still performs as well. Our glazing guide explains this in detail.
What else do the regulations cover?
- Structure: foundations, beams and lintels designed by an engineer.
- Fire: smoke alarms, escape windows from upper-floor rooms, and fire separation near boundaries.
- Ventilation: extract fans in kitchens and bathrooms and background ventilation.
- Drainage: new connections and any building near public sewers.
- Electrics: work certified by a registered electrician.
- Safety glazing: toughened or laminated glass in doors and low-level panes.
What happens at the end?
When the final inspection is passed, building control issues a completion certificate (in Scotland, the completion certificate is accepted by the council). Keep it with your property documents; you will need it when you sell.
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.