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 £65,500 to £81,000 including VAT, or £2,600 to £3,200 per m² at South West prices. Cornwall charges £1,140 for building control on a 10 to 40 m² extension (full plans, including VAT): the 7th dearest of the 91 councils we checked, 36% above the typical council (£840). With design, a structural engineer, building control and a 15% contingency, budget £81,500 to £105,000.
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 | £65,500 to £81,000 | £81,500 to £105,000 |
| Double-storey extension, 40 m² | £95,000 to £120,000 | £117,000 to £152,000 |
| Dormer loft conversion, 30 m² | £58,500 to £72,500 | £73,000 to £93,500 |
| 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 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, 40-150 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 charges £1,140 for building control on a 10 to 40 m² extension (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, Not stated (page last updated 8 May 2026). Inc VAT ("All fees include VAT").
Private approved inspectors 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
Own page. Extensions, lofts and garage conversions share floor-area rows (<=10, 10-40, 40-150 m2); loft figure is the 10-40 row; garage conversion priced on the same rows.
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.
Other councils in 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 £65,500 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 fees and a 15% contingency, budget £81,500 to £105,000.
How much is building control in Cornwall?
Cornwall charges £1,140 (including VAT) for a 10 to 40 m² extension (full plans), according to its published schedule (Not stated (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 £58,500 to £72,500 including VAT for the building work, and about £73,000 to £93,500 with fees 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
- Trades: price an extension element by element with editable unit rates