Budget check · September 2026
The hidden costs of an extension
A builder's quote usually covers the building work. On a typical 25 m² single-storey rear extension you should also budget £5,800 to £11,800 for the survey, design drawings, a structural engineer and building control, a 15% contingency of £9,500 to £11,700, and whatever the quote leaves out inside and outside the new room.
Fees and approvals
These sit outside almost every builder's quote. Building control figures are the middle half of charges at the councils we surveyed; your own council's charge may be higher or lower.
| Item | Typical cost | Notes |
|---|---|---|
| Measured survey | £600 to £1,800 | Usually needed |
| Design drawings (planning and building regulations) | £3,600 to £6,000 | Usually needed |
| Structural engineer | £900 to £3,000 | Usually needed |
| Building control (council, full plans) | £740 to £1,000 | Middle half of 91 councils in England and Wales for a 10 to 40 m² extension |
| Householder planning application (England) | £548 | Only if your project needs planning permission |
| Party wall surveyor (per neighbour) | £1,100 to £1,800 | Only if you build on or near a shared wall |
A contingency
Something always turns up once the ground is open or the old walls come down. We add 15% of the building cost as a planning allowance. If you do not spend it, it becomes a saving.
Inside the new room
- Flooring, skirting and internal doors
- Radiators or underfloor heating, and extending the heating system
- Lighting, extra sockets and any upgrade to the consumer unit
- Plastering and decorating, if the quote stops at first fix
- Blinds, curtains and furniture
- The kitchen itself, if the extension is for a new kitchen (see kitchen costs)
Services
- A bigger or new boiler if the extra space is more than the existing one can heat. A like-for-like combi swap is typically £2,500 to £3,200 (see boiler costs).
- Moving or protecting drains under the new footprint
- Permission from your water company if you build over or close to a public sewer
Outside
- Repairing the garden, lawn, patio and paths after the builders leave
- Fences, gates and access
- A permit from the council if the skip has to sit on the road
Living through the build
- Storage, or somewhere else to stay, while the kitchen or a bathroom is out of use
- Telling your home insurer before work starts
Add it all up
Get the building cost and fees for your size and region in the extension cost calculator, then list the extras above in the renovation budget planner. When quotes arrive, ask each builder to confirm which of these items are included.