Building regulations for an extension: what you must comply with
3 min
6
September 2026
Key figures in this guide
One-minute decision
What are you trying to work out?
Use the checker first, then turn the result into a clearer brief before speaking to professionals.
Open the checkerEvery house extension in the UK needs building regulations approval, even when planning permission is not required. Planning controls what the extension looks like and where it goes. Building regulations control whether it is safe, warm, dry and properly built.
Approval comes from your local authority building control or a private approved inspector, costs £700 - £1,900 in 2026, and ends with a completion certificate you will need when you sell.
The parts that usually apply to an extension
| Part | Covers | What it means in practice |
|---|---|---|
| A | Structure | Engineer calculations for foundations, beams and roof |
| B | Fire safety | Escape windows, protected stairs, fire doors on loft work |
| C | Damp and contaminants | Damp-proof course and membrane, ground conditions |
| H | Drainage | Building over or near drains, inspection chambers, water test |
| K | Stairs and protection | Stair pitch, headroom, guarding, glazing |
| L | Energy | Insulation levels, glazing area, air tightness |
| M | Access | Level threshold and door widths where reasonable |
| P | Electrical | Certified work by a Part P registered electrician |
| F | Ventilation | Extract fans, background ventilation, trickle vents |
Key 2026 requirements to design around
- Insulation targets: roughly 0.15 W/square metre K for roofs, 0.18 for walls, 0.13 for floors
- Windows and doors: 1.4 W/square metre K or better
- Glazing area in an extension limited to about 25 per cent of the new floor area plus the openings you cover up, unless you compensate elsewhere with SAP calculations
- Foundations typically 1 metre deep minimum, deeper near trees or made ground
- Escape window in every habitable room in a loft, with a protected stair route
- Mains-powered interlinked smoke alarms on every storey
- Building over or within 3 metres of a public sewer needs a build-over agreement from the water company
The process, step by step
- 01Get drawings and a specification, plus structural calculations.
- 02Submit a full plans application, or a building notice for simple work.
- 03Get the plan check comments and resolve them before starting.
- 04Give notice and book inspections at each stage.
- 05Provide electrical, gas and window certificates at the end.
- 06Receive the completion certificate and keep it with your deeds.
What catches people out
- Assuming permitted development means no approval is needed at all
- Large sliding doors that break the glazing allowance with no compensating measures
- Extending over a drain without a build-over agreement
- A loft conversion where the existing stair cannot be protected
- Starting foundations before the plan check comes back
- Electrical work done by an unregistered trade, which then needs re-testing
What it costs alongside
| Item | 2026 cost |
|---|---|
| Building control application and inspections | £700 - £1,900 |
| Structural engineer | £900 - £2,600 |
| SAP or glazing calculations | £250 - £600 |
| Drainage build-over agreement | £350 - £700 |
| Air tightness test, where required | £300 - £700 |
Build it into your scope
Most disputes about compliance are really disputes about what the builder priced. Renovome writes the regulatory requirements into your brief, so insulation, glazing, drainage and certificates are priced from the start rather than argued about at inspection. Create your project brief and compare builders on the same scope.
Last reviewed: September 2026
Community posts reflect individual experiences and opinions. They are not a substitute for advice from an appropriately qualified professional.