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 Southsea?
In Southsea, 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. Building costs in the South East run about 12% above the UK average, and the figures for Southsea are adjusted to reflect that. 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 |
Your quote will depend mainly on:
- Floor lowering and underpinning
- Existing cellar or new excavation
- Waterproofing system
- Lightwells
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 Southsea?
Approvals for a basement conversion in Southsea follow the rules for 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.
We check this for your address before design work starts, so there are no surprises later.
What we handle
What we take care of:
- Lightwells and windows
- Party wall notices
- Building control sign-off
- Insulation, ventilation and drainage pumps
- Structural and ground survey
- Underpinning or new structure where needed
How long does it take?
Build time is typically 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 Southsea
The South East has a broad mix of housing, from Victorian terraces in the towns to post-war semis and newer estates, so extension types vary street by street.
Southsea sits in Hampshire, about 16 miles south-east of Southampton. We cover the whole of Southsea, including postcodes PO4.
How it works
- 01
Free visit
We look at the property, talk through what you want and measure up. There is no charge and no obligation.
- 02
Drawings and written quote
We prepare the design and give you a written, itemised quote with inclusions and exclusions.
- 03
Approvals in place
We handle planning, building control and neighbour notices before work starts.
- 04
Build and sign-off
We run the build to a written programme and hand over the completion certificate.
Questions about basement conversions in Southsea
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 much does a basement conversion cost in Southsea?
In Southsea, 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.
How long does it take?
A cellar conversion takes around 8 to 16 weeks; new basements take longer.
Can a basement be a bedroom?
Yes, with a suitable escape route such as a lightwell window or protected stair.
Who approves a basement conversion in Southsea?
Planning matters in Southsea go to the council covering your address, and the work needs building control approval. We prepare and submit the applications for you.