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 Keynsham?
In Keynsham, 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 Keynsham 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 main things that change the cost:
- Lightwells
- Party wall surveyor fees
- Floor lowering and underpinning
- Existing cellar or new excavation
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 basement conversion in Keynsham?
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.
We check this for your address before design work starts, so there are no surprises later.
What we handle
The work we manage on a basement conversion:
- Lightwells and windows
- Structural and ground survey
- Party wall notices
- Insulation, ventilation and drainage pumps
- Underpinning or new structure where needed
- Waterproofing design (tanking or cavity drain membrane)
How long does it take?
Expect 8 to 16 weeks for a cellar conversion; longer for new excavation. Allow time beforehand for drawings and approvals, usually 3 to 5 months.
Building in Keynsham
The South West mixes stone and render cottages with post-war and newer estates, and local materials matter more here when matching an extension.
Our Keynsham coverage includes postcode districts BS30 and BS31, and we also work in nearby places such as Kingswood, Mangotsfield and Bath.
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 in place
We handle planning, building control and neighbour notices before work starts.
- 04
Construction and completion
We build, put right any snags and hand over the building control certificate.
Questions about basement conversions in Keynsham
Do I need party wall agreements?
In England and Wales, excavating near a neighbour’s foundations usually triggers the Party Wall etc. Act 1996.
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.
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 Keynsham?
In Keynsham, 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.
Who approves a basement conversion in Keynsham?
Planning matters in Keynsham go to the council covering your address, and the work needs building control approval. We prepare and submit the applications for you.