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 Hammersmith?
In Hammersmith, a basement conversion typically costs £2,450–£4,150 per m² for converting an existing cellar, including VAT at 20% (standard rate). For a typical 30 m² project that is roughly £73,500–£124,500. Prices across London are typically 30% above the UK average for this kind of work, which is reflected in the range for Hammersmith. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (21 m²) | £51,500–£87,000 |
| Typical project (30 m²) | £73,500–£124,500 |
| Larger project (42 m²) | £103,000–£174,500 |
| Rate per m² | £2,450–£4,150 |
The biggest cost drivers are:
- Floor lowering and underpinning
- Party wall surveyor fees
- Existing cellar or new excavation
- Ground water and access
For a figure based on your own measurements, try the guide price tool, then book a free home visit for a fixed quote.
Do you need planning permission for a basement conversion in Hammersmith?
The approval route for Hammersmith is set by planning law 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.
Our first visit includes a check of your planning history, so you know the route before paying for drawings.
What we handle
On a typical basement conversion we handle:
- Electrics, heating and plastering
- Insulation, ventilation and drainage pumps
- Lightwells and windows
- Structural and ground survey
- Waterproofing design (tanking or cavity drain membrane)
- Party wall notices
How long does it take?
Allow 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 Hammersmith
Victorian and Edwardian terraces dominate many London streets, which is why side return, wraparound and mansard projects are so common here.
If your home is in Hammersmith or close by in Chiswick, Fulham and Barnes, a free home visit is the first step.
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
Approvals
Planning or permitted development confirmation, building regulations and any party wall matters.
- 04
Build and sign-off
We run the build to a written programme and hand over the completion certificate.
Questions about basement conversions in Hammersmith
Who approves a basement conversion in Hammersmith?
Planning matters in Hammersmith go to the council covering your address, and the work needs building control approval. We prepare and submit the applications for you.
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.
How much does a basement conversion cost in Hammersmith?
In Hammersmith, a basement conversion typically costs £2,450–£4,150 per m² for converting an existing cellar, including VAT at 20% (standard rate). A typical 30 m² project comes to roughly £73,500–£124,500. Your fixed price depends on the survey and drawings.
Is new excavation worth it?
It can be in high-value areas, but costs are high. We give a feasibility view before design starts.