What is a single-storey extension?
A single-storey extension is a ground-floor addition, usually at the rear of a house, that enlarges a kitchen, living or dining space. Most are built with cavity walls on concrete foundations and finished with a flat roof and roof lantern or a pitched tiled roof.
How much does a single-storey extension cost in South Glamorgan?
In South Glamorgan, a single-storey extension typically costs £1,800–£2,650 per m² of floor area, including VAT at 20% (standard rate). For a typical 20 m² project that is roughly £36,000–£53,000. 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 (14 m²) | £25,000–£37,000 |
| Typical project (20 m²) | £36,000–£53,000 |
| Larger project (28 m²) | £50,500–£74,000 |
| Rate per m² | £1,800–£2,650 |
The main things that change the cost:
- Glazing: sliding and bifold doors cost more than French doors
- Floor area and the length of new wall
- Kitchen, flooring and finishes, which we can include or leave to you
- How much of the existing house is opened up
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 single-storey extension in South Glamorgan?
The approval route for South Glamorgan is set by planning law in Wales. In Wales a single-storey rear extension is often permitted development: up to 4 metres beyond the rear wall and up to 4 metres high, with eaves no higher than 3 metres within 2 metres of a boundary and no more than half the land around the original house covered. Wales has no larger home extension prior approval route, so deeper extensions need a planning application, as do extensions to flats, listed buildings and houses whose rights have been removed.
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.
If the answer is not clear-cut, we can apply for formal confirmation before you commit to the build.
What we handle
Handled as part of most single-storey extensions:
- Cavity walls, insulation and roof to current building regulations
- Structural calculations for steels and lintels
- Knock-through to the existing house with steel beams
- Building control inspections and completion certificate
- Foundations, drainage and any build-over agreement
- Doors, windows and roof glazing installed
How long does it take?
Expect 10 to 14 weeks on site for a typical rear extension, after approvals. Before that, surveys, drawings and approvals usually take 6 to 12 weeks, longer if the council asks for changes.
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
Visit and measured survey
A free visit to understand the project, check access and take measurements.
- 02
Drawings and written quote
We prepare the design and give you a written, itemised quote with inclusions and exclusions.
- 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 single-storey extensions in South Glamorgan
What is a build-over agreement?
If a public sewer runs within 3 metres of the extension, the water company normally has to agree to the work before it starts. We check sewer records during design and apply where needed.
Is VAT charged on an extension?
Yes. Building work on an existing home is standard-rated, so VAT at 20% applies. Our guide prices include it.
Do I need planning permission for a single-storey extension in South Glamorgan?
In Wales a single-storey rear extension is often permitted development: up to 4 metres beyond the rear wall and up to 4 metres high, with eaves no higher than 3 metres within 2 metres of a boundary and no more than half the land around the original house covered. Wales has no larger home extension prior approval route, so deeper extensions need a planning application, as do extensions to flats, listed buildings and houses whose rights have been removed.
Can I stay in the house during the build?
Usually yes. We build the shell before opening up the back wall, so the house stays weathertight for most of the job. There will be a few weeks when the kitchen or back room is out of use.
How long does a single-storey extension take in South Glamorgan?
Typically 10 to 14 weeks on site for a typical rear extension, after approvals. Design and approvals beforehand usually take 6 to 12 weeks.
How much does a single-storey extension cost in South Glamorgan?
In South Glamorgan, a single-storey extension typically costs £1,800–£2,650 per m² of floor area, including VAT at 20% (standard rate). A typical 20 m² project comes to roughly £36,000–£53,000. Your fixed price depends on the survey and drawings.