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.
Because there is only one floor to support, a single-storey extension is the most common way to add space. Many small rear extensions can be built under permitted development; larger or more prominent designs need a planning application.
What we handle
- Measured survey and design drawings
- Structural calculations for steels and lintels
- Foundations, drainage and any build-over agreement
- Cavity walls, insulation and roof to current building regulations
- Doors, windows and roof glazing installed
- Knock-through to the existing house with steel beams
- First and second fix electrics, heating and plastering
- Building control inspections and completion certificate
Options to consider
Flat roof with lantern
A warm-deck flat roof keeps the ceiling high and lets a lantern bring light into the middle of the room.
Pitched or lean-to roof
A tiled roof can match the house and suits a traditional look; roof windows add daylight.
Sliding, bifold or French doors
Wide openings link the room to the garden. Sliding doors have slimmer frames; bifolds open the full width.
Open-plan knock-through
Taking out the old back wall joins the extension to the house. A steel beam carries the load above.
How much does a single-storey extension cost?
A single-storey extension typically costs £1,950–£2,900 per m² of floor area across the UK, including VAT at 20% (standard rate). A typical 20 m² project comes to roughly £39,000–£58,000. Build time is usually 10 to 14 weeks on site for a typical rear extension, after approvals. Design, structural calculations and council fees are extra.
| Region | Guide range (per m² of floor area) |
|---|---|
| London | £2,550–£3,750 |
| South East | £2,200–£3,250 |
| East of England | £2,050–£3,050 |
| South West | £2,000–£2,950 |
| West Midlands | £1,900–£2,800 |
| East Midlands | £1,850–£2,750 |
| North West | £1,850–£2,750 |
| Yorkshire and the Humber | £1,800–£2,700 |
| North East | £1,750–£2,600 |
| Wales | £1,800–£2,650 |
| Scotland | £1,900–£2,800 |
| Northern Ireland | £1,700–£2,500 |
What changes the price:
- 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
- How much of the existing house is opened up
- Kitchen, flooring and finishes, which we can include or leave to you
Use the guide price tool for a figure based on your measurements, or see all guide prices.
Do I need planning permission?
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.
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.
Scotland
In Scotland a single-storey extension is often permitted development (Class 1A): where any part is within 1 metre of a boundary, it can project up to 3 metres from the rear wall of a terraced house or 4 metres on other houses, with eaves no higher than 3 metres, an overall height of 4 metres and no more than half the front or rear garden covered. The rights do not apply in conservation areas or to flats, and a building warrant will usually still be needed before work starts.
Northern Ireland
In Northern Ireland the Planning (General Permitted Development) Order (Northern Ireland) 2015 allows single-storey rear extensions of up to 3 metres on attached houses and 4 metres on detached houses, no higher than 4 metres, with eaves no higher than 3 metres within 2 metres of a boundary. Houses with removed rights need planning permission, listed buildings need consent, and in conservation areas extensions must stay single-storey and must not come forward of the front or side walls.
Even when planning permission is not needed, building regulations approval (a building warrant in Scotland) usually is. We make the applications and book the inspections.
How long does it take?
Typically 10 to 14 weeks on site for a typical rear extension, after approvals. Surveys, design and approvals usually take 6 to 12 weeks beforehand.
How it works
- 01
Free visit
We look at the property, discuss what you want and measure up.
- 02
Design and fixed quote
Drawings, calculations and an itemised price for the agreed specification.
- 03
Approvals
Planning or permitted development, building regulations and neighbour matters.
- 04
Build and sign-off
Built to a written programme, with a completion certificate at the end.
Single-storey extensions: questions and answers
Do I need planning permission for a single-storey extension?
Not always. Many small rear extensions are permitted development, within limits on depth, height and how much of the garden is covered. The limits depend on which nation you live in and on protected areas such as conservation areas, and flats or houses whose rights have been removed need an application. We check this at the survey and can apply to the council for formal confirmation.
How long does a single-storey extension take?
Once approvals are in place, a typical rear extension takes 10 to 14 weeks on site. Design and approvals usually take another 6 to 12 weeks beforehand, depending on whether a planning application is needed.
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.
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.
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.