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 Mendip?
In Mendip, 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. Prices in the South West sit near the national average for this kind of work, and the range for Mendip reflects that. 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 main things that change the cost:
- Floor lowering and underpinning
- Party wall surveyor fees
- Existing cellar or new excavation
- Waterproofing system
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 Mendip?
Whether you need planning permission depends on your house and its location 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.
If the answer is not clear-cut, we can apply for formal confirmation before you commit to the build.
What we handle
Handled as part of most basement conversions:
- Underpinning or new structure where needed
- Party wall notices
- Insulation, ventilation and drainage pumps
- Structural and ground survey
- Building control sign-off
- Lightwells and windows
How long does it take?
Expect 8 to 16 weeks for a cellar conversion; longer for new excavation. Design and approvals come first and typically take 3 to 5 months, depending on the approval route.
Building in Mendip
The South West mixes stone and render cottages with post-war and newer estates, and local materials matter more here when matching an extension.
Mendip sits in Somerset, about 26 miles north-east of Taunton. We cover the whole of Mendip, including postcodes BA5.
How it works
- 01
Survey and brief
We visit, discuss your ideas and budget, and survey the property or site at no cost.
- 02
Design and pricing
Design options, then a fixed quote based on the chosen drawings and specification.
- 03
Approvals
Planning or permitted development confirmation, building regulations and any party wall matters.
- 04
Build and handover
Work follows the agreed programme, with inspections along the way and a completion certificate at the end.
Questions about basement conversions in Mendip
Can a basement be a bedroom?
Yes, with a suitable escape route such as a lightwell window or protected stair.
How much does a basement conversion cost in Mendip?
In Mendip, 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.
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.
Do you build basement conversions in Mendip?
Yes. We build basement conversions across Mendip, including postcodes BA5. Every project starts with a free home visit.
Is new excavation worth it?
It can be in high-value areas, but costs are high. We give a feasibility view before design starts.