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 Welwyn Garden City?
In Welwyn Garden City, a single-storey extension typically costs £2,050–£3,050 per m² of floor area, including VAT at 20% (standard rate). For a typical 20 m² project that is roughly £41,000–£61,000. The guide for Welwyn Garden City reflects typical costs in the East of England, roughly 6% above the UK-wide figure. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (14 m²) | £28,500–£42,500 |
| Typical project (20 m²) | £41,000–£61,000 |
| Larger project (28 m²) | £57,500–£85,500 |
| Rate per m² | £2,050–£3,050 |
What moves the price most:
- Floor area and the length of new wall
- Ground conditions and foundation depth
- Roof type and the size of any lantern or roof windows
- Glazing: sliding and bifold doors cost more than French doors
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 single-storey extension in Welwyn Garden City?
Here is how the rules work for a single-storey extension in England. In England a single-storey rear extension is often permitted development: broadly up to 3 metres deep on a semi-detached or terraced house and 4 metres on a detached house, no higher than 4 metres, with eaves no higher than 3 metres within 2 metres of a boundary, and covering no more than half the garden. The larger home extension scheme allows up to 6 or 8 metres if you notify the council first and neighbours have a chance to comment. In conservation areas, National Parks and National Landscapes the larger home extension route, side extensions and cladding are not permitted development. Flats and houses whose rights have been removed need a planning application, and listed buildings need listed building consent.
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.
If the answer is not clear-cut, we can apply for formal confirmation before you commit to the build.
What we handle
On a typical single-storey extension we handle:
- Cavity walls, insulation and roof to current building regulations
- First and second fix electrics, heating and plastering
- Knock-through to the existing house with steel beams
- Structural calculations for steels and lintels
- Building control inspections and completion certificate
- Doors, windows and roof glazing installed
How long does it take?
Allow 10 to 14 weeks on site for a typical rear extension, after approvals. Design and approvals come first and typically take 6 to 12 weeks, depending on the approval route.
Building in Welwyn Garden City
Across the East of England, 1930s and post-war semis, newer estates and rural cottages all feature, each with different extension and loft options.
Our Welwyn Garden City coverage includes postcode districts AL8, and we also work in nearby places such as Hatfield, Hertford and St Albans.
How it works
- 01
Survey and brief
We visit, discuss your ideas and budget, and survey the property or site at no cost.
- 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 Welwyn Garden City
Do you build single-storey extensions in Welwyn Garden City?
Yes. We build single-storey extensions across Welwyn Garden City, including postcodes AL8. Every project starts with a free home visit.
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 much does a single-storey extension cost in Welwyn Garden City?
In Welwyn Garden City, a single-storey extension typically costs £2,050–£3,050 per m² of floor area, including VAT at 20% (standard rate). A typical 20 m² project comes to roughly £41,000–£61,000. Your fixed price depends on the survey and drawings.
Do I need a party wall agreement?
If you are in England or Wales and the work involves a shared wall, building on the boundary or digging foundations close to a neighbour, the Party Wall etc. Act 1996 applies and notices must be served before work starts. We advise on this at the design stage.
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.