Local costs · South West · September 2026
How much does an extension cost in Cornwall?
A mid-range 25 m² single-storey extension in Cornwall costs about £66,000 to £81,000 including VAT, or £2,600 to £3,200 per m² at South West prices. Cornwall’s own published charge for building control on a 10 to 40 m² extension is £1,140 (full plans, including VAT): the 7th dearest of the 91 councils we checked, 36% above the typical council (£840). With a measured survey, building regulations drawings, a structural engineer, Cornwall’s own building control charge and a 15% contingency, plan for £81,000 to £102,000.
Work out your project in Cornwall
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Typical prices in Cornwall
| Project | Building work | Budget to plan for |
|---|---|---|
| Single-storey extension, 25 m² | £66,000 to £81,000 | £81,000 to £102,000 |
| Double-storey extension, 40 m² | £95,000 to £120,000 | £115,000 to £149,000 |
| Dormer loft conversion, 30 m² | £49,000 to £60,500 | £60,500 to £78,000 |
| Kitchen, medium | £9,200 to £16,300 | £10,600 to £18,800 |
| Bathroom refit | £6,100 to £9,200 | £7,100 to £10,600 |
Building prices in the South West run about 4% above the UK average outside London. Kitchens and bathrooms vary less from place to place than extensions and lofts.
Building control fees in Cornwall
| Work | Charge |
|---|---|
| Extension up to 10 m² (full plans) | £660 |
| Extension 10 to 40 m² (full plans) | £1,140 |
| Larger extension, 40 to 150 m² (full plans) | £1,398 |
| Extension 10 to 40 m² (building notice) | £1,248 |
| Loft conversion up to 40 m² | £1,140 |
| Garage conversion | Not published |
Cornwall’s own published charge for building control on a 10 to 40 m² extension is £1,140 (full plans, including VAT): the 7th dearest of the 91 councils we checked, 36% above the typical council (£840). Source: Cornwall’s building control charges. The schedule doesn’t give a start date (page last updated 8 May 2026). The charges include VAT.
Private building control firms (registered building control approvers) set their own prices and are often competitive, so it is worth asking one or two for a quote as well. See how Cornwall compares with every council we checked.
Notes on Cornwall’s schedule
Cornwall publishes its charges on its own page. Extensions, loft conversions and garage conversions use the same charges by floor area: up to 10 m², 10 to 40 m² and 40 to 150 m². The loft figure is the 10 to 40 m² charge.
Planning
If your extension needs planning permission, a householder application in England costs £548. Many single-storey rear extensions are permitted development and need no application, and a lawful development certificate (£274) confirms it in writing.
Other councils in the South West
Building control for a 10 to 40 m² extension:
- Bath and North East Somerset: £696
- Bristol: £735
- Wiltshire: £755
- Exeter: £800
- Mid Devon: £896
- North Devon: £896
- Plymouth: £973
- South Hams: £1,095
- Teignbridge: £1,095
- West Devon: £1,095
Questions people ask
How much does an extension cost in Cornwall?
A mid-range 25 m² single-storey extension in Cornwall costs about £66,000 to £81,000 including VAT for the building work, or £2,600 to £3,200 per m², based on South West prices reviewed in September 2026. With a measured survey, design, a structural engineer, Cornwall’s own building control charge and a 15% contingency, plan for £81,000 to £102,000.
How much is building control in Cornwall?
Cornwall’s own published charge for full plans approval of a 10 to 40 m² extension is £1,140 (including VAT). The schedule doesn’t give a start date (page last updated 8 May 2026).
How much does a loft conversion cost in Cornwall?
A mid-range 30 m² dormer loft conversion in Cornwall costs about £49,000 to £60,500 including VAT for the building work, and about £60,500 to £78,000 with fees, Cornwall’s own building control charge and a 15% contingency.
Before you get quotes
- How to compare builders’ quotes line by line
- Plan your whole budget, including the costs people forget
- When quotes arrive, check each one against the typical range for your area
Work in Cornwall? Firm listings on Estiquote’s map of checked local firms open soon: see what you’ll need. Listing will be free while we build the map, with at least 30 days’ notice before any charge.