Extension cost by type
Typical all-in cost including build and standard finish, before kitchen fit-out.
| Extension type | Rough size | Typical cost |
|---|---|---|
| Small single-storey rear | 10 to 15 m² | £20,000 to £45,000 |
| Large single-storey / kitchen | 20 to 30 m² | £40,000 to £80,000 |
| Double-storey extension | 30 to 40 m² | £55,000 to £120,000 |
| Wrap-around extension | 30 to 50 m² | £60,000 to £130,000+ |
London and the South East sit at the top of these ranges; add for kitchens, bathrooms and glazing.
Extension cost per square metre
A useful sense-check once you know the floor area you want to add.
| Standard of finish | Cost per m² | Example: 25 m² extension |
|---|---|---|
| Basic / budget | £1,500 to £1,900 | £37,500 to £47,500 |
| Mid-range | £1,900 to £2,400 | £47,500 to £60,000 |
| High specification | £2,400 to £3,000+ | £60,000 to £75,000+ |
Per-m² rates cover the build; kitchens, bathrooms and premium glazing are extra.
What drives the cost
Why extension quotes vary so much:
- Size and number of storeys, cost rises with floor area and height.
- Ground conditions and foundations, sloping or poor ground costs more.
- Glazing, bi-fold doors, roof lanterns and large windows are expensive.
- Fit-out, a kitchen or bathroom in the extension adds significantly.
- Structural work, steels and knocking through to existing rooms.
- Location and access, and professional and planning fees.
Costs beyond the build
Budget for these on top of the per-m² build cost:
- Architect and structural engineer fees, often 8 to 15% of build cost.
- Planning application and building control fees.
- Party wall surveyor fees where neighbours are affected.
- Kitchen, bathroom and internal fit-out.
- Redecorating and flooring in the new and adjoining rooms.
- A 10 to 20% contingency for the unexpected.
Planning and permitted development
Many single-storey rear extensions fall under permitted development within size limits, but larger, double-storey, front or side extensions, and homes in conservation areas usually need planning permission. All extensions need building regulations approval, and party wall agreements are common.
Set your buying budget before you extend
An extension is a large commitment on top of your purchase. Our planner helps you fix the full cost of buying and the reserve you should keep first, so you know what is realistically left for a project like an extension.