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.
Converting an existing cellar is often possible without planning permission, while a new basement or a lightwell visible from the street usually needs an application.
What we handle
- Structural and ground survey
- Waterproofing design (tanking or cavity drain membrane)
- Underpinning or new structure where needed
- Lightwells and windows
- Insulation, ventilation and drainage pumps
- Electrics, heating and plastering
- Party wall notices
- Building control sign-off
Options to consider
Cellar conversion
Uses existing space; may need lowering of the floor.
New basement
Excavation under the house or garden; a major structural project.
Lightwell
A glazed or open lightwell brings daylight and an escape route.
Cinema or gym
Rooms that benefit from little natural light.
How much does a basement conversion cost?
A basement conversion typically costs £1,900–£3,200 per m² for converting an existing cellar across the UK, including VAT at 20% (standard rate). A typical 30 m² project comes to roughly £57,000–£96,000. Build time is usually 8 to 16 weeks for a cellar conversion; longer for new excavation. Design, structural calculations and council fees are extra.
| Region | Guide range (per m² for converting an existing cellar) |
|---|---|
| London | £2,450–£4,150 |
| South East | £2,150–£3,600 |
| East of England | £2,000–£3,400 |
| South West | £1,950–£3,250 |
| West Midlands | £1,850–£3,100 |
| East Midlands | £1,800–£3,050 |
| North West | £1,800–£3,000 |
| Yorkshire and the Humber | £1,750–£3,000 |
| North East | £1,700–£2,900 |
| Wales | £1,750–£2,950 |
| Scotland | £1,850–£3,100 |
| Northern Ireland | £1,650–£2,750 |
What changes the price:
- Existing cellar or new excavation
- Floor lowering and underpinning
- Waterproofing system
- Lightwells
- Ground water and access
- Party wall surveyor fees
Use the guide price tool for a figure based on your measurements, or see all guide prices.
Do I need planning permission?
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.
Wales
In Wales converting an existing cellar usually does not need planning permission; new excavation and lightwells may.
Scotland
In Scotland converting an existing cellar may not need planning permission, but a building warrant is needed, and new excavation usually needs planning permission.
Northern Ireland
In Northern Ireland converting an existing cellar usually does not need planning permission; new excavation and external changes may.
Even when planning permission is not needed, building regulations approval (a building warrant in Scotland) usually is. We make the applications and book the inspections.
How long does it take?
Typically 8 to 16 weeks for a cellar conversion; longer for new excavation. Surveys, design and approvals usually take 3 to 5 months beforehand.
How it works
- 01
Free visit
We look at the property, discuss what you want and measure up.
- 02
Design and fixed quote
Drawings, calculations and an itemised price for the agreed specification.
- 03
Approvals
Planning or permitted development, building regulations and neighbour matters.
- 04
Build and sign-off
Built to a written programme, with a completion certificate at the end.
Basement conversions: questions and answers
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.
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 I need party wall agreements?
In England and Wales, excavating near a neighbour’s foundations usually triggers the Party Wall etc. Act 1996.
Can a basement be a bedroom?
Yes, with a suitable escape route such as a lightwell window or protected stair.
How long does it take?
A cellar conversion takes around 8 to 16 weeks; new basements take longer.
Is new excavation worth it?
It can be in high-value areas, but costs are high. We give a feasibility view before design starts.