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 Horley?
In Horley, a basement conversion typically costs £2,150–£3,600 per m² for converting an existing cellar, including VAT at 20% (standard rate). For a typical 30 m² project that is roughly £64,500–£108,000. We start from a UK-wide guide and adjust it for the South East, where labour and materials are typically about 12% above average. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (21 m²) | £45,000–£75,500 |
| Typical project (30 m²) | £64,500–£108,000 |
| Larger project (42 m²) | £90,500–£151,000 |
| Rate per m² | £2,150–£3,600 |
The biggest cost drivers are:
- Waterproofing system
- Lightwells
- Party wall surveyor fees
- Floor lowering and underpinning
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 Horley?
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.
Where work involves a shared wall, building on the boundary, or digging foundations within 3 metres of a neighbour’s building and deeper than its foundations (or within 6 metres for deeper work), the Party Wall etc. Act 1996 applies and notices must be served before work starts.
At the survey we check your house’s planning history and any restrictions, and tell you which route applies.
What we handle
The work we manage on a basement conversion:
- Electrics, heating and plastering
- Party wall notices
- Waterproofing design (tanking or cavity drain membrane)
- Insulation, ventilation and drainage pumps
- Underpinning or new structure where needed
- Building control sign-off
How long does it take?
The build itself usually takes 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 Horley
Commuter towns across the South East have many 1930s and post-war semis that suit rear extensions and hip-to-gable loft conversions.
Horley sits in Surrey, about 18 miles east of Guildford. We cover the whole of Horley, including postcodes RH6.
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 basement conversions in Horley
Do you build basement conversions in Horley?
Yes. We build basement conversions across Horley, including postcodes RH6. Every project starts with a free home visit.
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 much does a basement conversion cost in Horley?
In Horley, a basement conversion typically costs £2,150–£3,600 per m² for converting an existing cellar, including VAT at 20% (standard rate). A typical 30 m² project comes to roughly £64,500–£108,000. Your fixed price depends on the survey and drawings.
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.