What is a side extension?
A side extension builds into the space beside a semi-detached or detached house, often over a driveway or side passage. It can be single-storey, for a utility room, downstairs toilet or study, or two-storey to add a bedroom above.
How much does a side extension cost in Cornwall?
In Cornwall, a side extension typically costs £1,750–£2,700 per m² of floor area, including VAT at 20% (standard rate). For a typical 28 m² project that is roughly £49,000–£75,500. Building costs in the South West are close to the UK average, so the figures for Cornwall are close to our national guide. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (20 m²) | £35,000–£54,000 |
| Typical project (28 m²) | £49,000–£75,500 |
| Larger project (39 m²) | £68,000–£105,500 |
| Rate per m² | £1,750–£2,700 |
The main things that change the cost:
- Boundary walls and access for materials
- Drains, gas meters or soil pipes that need moving
- Single or two storeys
- Steel work where the side wall is opened
A guide range is only a starting point: the guide price tool gives an instant figure for your size, and a free survey turns it into a fixed quote.
Do you need planning permission for a side extension in Cornwall?
The approval route for Cornwall is set by planning law in England. In England a single-storey side extension can be permitted development if it is no more than 4 metres high and no wider than half the width of the original house. Side extensions of more than one storey always need planning permission, as do side extensions in conservation areas, National Parks and National Landscapes.
Building regulations approval comes from the council’s building control service or a registered building control approver. The work is inspected at key stages, such as foundations, drainage and insulation, and a completion certificate is issued at the end.
At the survey we check your house’s planning history and any restrictions, and tell you which route applies.
What we handle
From survey to sign-off, the job normally involves:
- Building control completion
- Planning drawings and application
- Structural calculations
- Opening into the existing house
- Foundations and drainage alterations
- Survey of the boundary, drains and access
How long does it take?
Allow 10 to 16 weeks on site depending on size and storeys. Allow time beforehand for drawings and approvals, usually 3 to 4 months.
Building in Cornwall
In parts of the South West, protected landscapes and conservation areas shape what can be built, so materials and roof forms are chosen carefully.
Towns we cover in Cornwall include Redruth, Falmouth, St Austell, Truro, Camborne and Newquay.
How it works
- 01
Visit and measured survey
A free visit to understand the project, check access and take measurements.
- 02
Design and pricing
Design options, then a fixed quote based on the chosen drawings and specification.
- 03
Permissions sorted
Applications, building control and neighbour arrangements, all before the start date.
- 04
Build and sign-off
We run the build to a written programme and hand over the completion certificate.
Questions about side extensions in Cornwall
Will I lose my driveway?
A side extension often takes the side drive. We look at whether parking can be kept at the front and whether a dropped kerb is needed.
Will a two-storey side extension look odd?
Not if it is designed well. Setting it back from the front and keeping the roof lower than the main ridge usually gives a natural look and is what most councils ask for.
Can a side extension go over a drain?
Often yes, with a build-over agreement from the water company or by rerouting the drain. We check this during the survey.
How much does a side extension cost in Cornwall?
In Cornwall, a side extension typically costs £1,750–£2,700 per m² of floor area, including VAT at 20% (standard rate). A typical 28 m² project comes to roughly £49,000–£75,500. Your fixed price depends on the survey and drawings.
Who approves a side extension in Cornwall?
Planning matters in Cornwall go to the council covering your address, and the work needs building control approval. We prepare and submit the applications for you.