What is a basement conversion?
A basement conversion turns an existing cellar into living space, or excavates a new basement under the house or garden. Waterproofing, structural support and daylight are the main design challenges.
How much does a basement conversion cost in Dorset?
In Dorset, a basement conversion typically costs £1,950–£3,250 per m² for converting an existing cellar, including VAT at 20% (standard rate). For a typical 30 m² project that is roughly £58,500–£97,500. Building costs in the South West are close to the UK average, so the figures for Dorset are close to our national guide. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (21 m²) | £41,000–£68,000 |
| Typical project (30 m²) | £58,500–£97,500 |
| Larger project (42 m²) | £82,000–£136,500 |
| Rate per m² | £1,950–£3,250 |
The biggest cost drivers are:
- Existing cellar or new excavation
- Floor lowering and underpinning
- Party wall surveyor fees
- Lightwells
Your exact price depends on the survey and drawings. The guide price tool gives a quick range in the meantime.
Do you need planning permission for a basement conversion in Dorset?
Here is how the rules work for a basement conversion in England. In England converting an existing cellar with no external change usually does not need planning permission. New excavation, lightwells and changes to the front often do, and some councils, particularly in London, have specific basement policies.
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.
We check this for your address before design work starts, so there are no surprises later.
What we handle
What we take care of:
- Insulation, ventilation and drainage pumps
- Lightwells and windows
- Underpinning or new structure where needed
- Structural and ground survey
- Waterproofing design (tanking or cavity drain membrane)
- Electrics, heating and plastering
How long does it take?
Expect 8 to 16 weeks for a cellar conversion; longer for new excavation. Before that, surveys, drawings and approvals usually take 3 to 5 months, longer if the council asks for changes.
Building in Dorset
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 Dorset include Bournemouth, Poole, Weymouth and Christchurch.
How it works
- 01
Visit and measured survey
A free visit to understand the project, check access and take measurements.
- 02
Design and fixed quote
Drawings, structural calculations and an itemised, fixed price for the agreed specification.
- 03
Permissions sorted
Applications, building control and neighbour arrangements, all before the start date.
- 04
Construction and completion
We build, put right any snags and hand over the building control certificate.
Questions about basement conversions in Dorset
How much does a basement conversion cost in Dorset?
In Dorset, a basement conversion typically costs £1,950–£3,250 per m² for converting an existing cellar, including VAT at 20% (standard rate). A typical 30 m² project comes to roughly £58,500–£97,500. Your fixed price depends on the survey and drawings.
Does a basement conversion need planning permission?
Converting an existing cellar without changing the outside usually does not. New excavation, lightwells and changes visible from outside usually do.
Can a basement be a bedroom?
Yes, with a suitable escape route such as a lightwell window or protected stair.
How do you keep a basement dry?
With a waterproofing system designed to BS 8102, typically a cavity drain membrane with a sump pump, or a tanking system.
How long does a basement conversion take in Dorset?
Typically 8 to 16 weeks for a cellar conversion; longer for new excavation. Design and approvals beforehand usually take 3 to 5 months.