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 South Glamorgan?
In South Glamorgan, a basement conversion typically costs £1,750–£2,950 per m² for converting an existing cellar, including VAT at 20% (standard rate). For a typical 30 m² project that is roughly £52,500–£88,500. Building costs in Wales run about 8% below the UK average, and the figures for South Glamorgan are adjusted to reflect that. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (21 m²) | £37,000–£62,000 |
| Typical project (30 m²) | £52,500–£88,500 |
| Larger project (42 m²) | £73,500–£124,000 |
| Rate per m² | £1,750–£2,950 |
What moves the price most:
- Existing cellar or new excavation
- Lightwells
- Waterproofing system
- Party wall surveyor fees
Use the guide price tool to see how size and specification change the figure, then ask for a written, itemised quote.
Do you need planning permission for a basement conversion in South Glamorgan?
The approval route for South Glamorgan is set by planning law in Wales. In Wales converting an existing cellar usually does not need planning permission; new excavation and lightwells may.
The Party Wall etc. Act 1996 applies in Wales, so notices must be served before work on a shared wall, on the boundary, or excavation close to a neighbour’s foundations.
We check this for your address before design work starts, so there are no surprises later.
What we handle
On a typical basement conversion we handle:
- Underpinning or new structure where needed
- Lightwells and windows
- Building control sign-off
- Party wall notices
- Electrics, heating and plastering
- Structural and ground survey
How long does it take?
The build itself usually takes 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 South Glamorgan
Wales has many terraced valley streets, stone cottages and post-war estates, so extension designs are shaped by slopes, materials and local character.
Towns we cover in South Glamorgan include Cardiff, Barry and Penarth.
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
Permissions sorted
Applications, building control and neighbour arrangements, all before the start date.
- 04
Construction and completion
We build, put right any snags and hand over the building control certificate.
Questions about basement conversions in South Glamorgan
Who approves a basement conversion in South Glamorgan?
Planning matters in South Glamorgan 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 planning permission for a basement conversion in South Glamorgan?
In Wales converting an existing cellar usually does not need planning permission; new excavation and lightwells may.
How much does a basement conversion cost in South Glamorgan?
In South Glamorgan, a basement conversion typically costs £1,750–£2,950 per m² for converting an existing cellar, including VAT at 20% (standard rate). A typical 30 m² project comes to roughly £52,500–£88,500. Your fixed price depends on the survey and drawings.
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.
Is new excavation worth it?
It can be in high-value areas, but costs are high. We give a feasibility view before design starts.
How long does it take?
A cellar conversion takes around 8 to 16 weeks; new basements take longer.