Updated 26 October 2026 · SK Home Improvements
A house extension adds new floor space to your home. In 2026 a single-storey extension typically costs £35,000–£75,000 and a double-storey extension £60,000–£120,000, both including VAT. Size, ground conditions, glazing and the finish inside drive the price, and fees for drawings and approvals usually come on top.
How much does a house extension cost in 2026?
A single-storey extension typically costs £35,000–£75,000 in 2026 and a double-storey extension typically costs £60,000–£120,000, both including VAT at 20%. These are guide ranges for building work on an existing house, not quotes. Where your project lands depends on its size, how complicated the structure is, the ground it sits on and the finish you want inside.
A small, simple rear extension with standard doors and a plain flat or pitched roof sits towards the lower end. A wide extension with a long run of glazing, structural steelwork, roof lanterns, underfloor heating and a new kitchen can sit at or beyond the top. Our guide-price tool gives a range based on your answers, and after a free site survey we confirm a fixed price for the agreed specification in writing.
What does an extension cost per square metre?
Many single-storey extensions in 2026 work out at roughly £2,000 to £3,200 per square metre of new floor area for the building work, including VAT. That usually covers foundations, walls, roof, windows and doors, insulation, plastering and basic electrics and heating, but not a new kitchen, flooring or decoration. Costs per square metre tend to be higher in London and the South East, and on small projects where fixed costs such as design, approvals and site set-up are spread over fewer square metres.
A double-storey extension usually costs less per square metre than a single-storey one, because the foundations and roof serve two floors, but the total is higher. These figures are typical and vary from job to job.
| Project | Typical new floor area | Main cost drivers |
|---|---|---|
| Small rear kitchen extension | 10–18 m² | Glazing, roof type, kitchen |
| Wide rear or wraparound extension | 20–40 m² | Steelwork, spans, roof lights |
| Two-storey side or rear extension | 25–50 m² over two floors | Roof tie-in, stair changes, bathroom |
What makes an extension cost more or less?
The biggest cost drivers are the ground, the structure and the glazing. Everything below floor level is hard to predict until the ground is opened up, which is why a proper survey matters.
- Foundations: clay soil, nearby trees or soft ground can mean deeper or piled foundations instead of standard trench footings.
- Drains: building over or near a public sewer usually needs the water company's agreement and sometimes a diversion.
- Openings and steel: removing most of the back wall of the house needs steel beams designed by a structural engineer.
- Glazing: bifold or sliding doors, large fixed panes and roof lanterns cost more than standard windows and doors.
- Roof: a pitched roof with tiles to match the house usually costs more than a flat roof, and a vaulted ceiling adds structure.
- Access: on a terraced house without side access, materials and waste may have to go through the house, which adds labour.
- Inside: underfloor heating, a new kitchen, a utility room or a downstairs toilet all add cost.
What other costs should you budget for?
On top of the building work, most extensions involve professional fees and approval costs. Some may already be in a builder's price, so check what is included. The figures below are typical and vary by area and project.
| Item | Typical cost |
|---|---|
| Measured survey and architectural drawings | £1,500–£4,500 |
| Structural engineer's calculations | £500–£1,500 |
| Planning application fee, if needed | Set nationally; differs between England, Wales, Scotland and Northern Ireland |
| Building control fees or building warrant fee | Often £800–£2,000, set by each council or body |
| Party wall surveyors (England and Wales only) | Often £1,000–£2,000 per neighbour |
| Water company build-over agreement | Fee set by the water company |
We also suggest keeping a contingency of around 10 to 15% of the build cost for things that cannot be seen until work starts. Our party wall guide explains when neighbours need formal notice.
Do you need planning permission for an extension?
Many extensions can be built without a full planning application under permitted development rights, but the limits depend on where in the UK you live, the type of house and whether it is listed or in a conservation area. In England, for example, a single-storey rear extension can usually project up to 3 m from the original rear wall of a semi-detached or terraced house and up to 4 m on a detached house, or up to 6 m and 8 m through the prior approval process. Wales, Scotland and Northern Ireland set their own rules.
Planning and Building Regulations are separate. Almost every extension needs Building Regulations approval, or a building warrant in Scotland, even where no planning application is needed. Our guides to planning permission and Building Regulations explain both, and we check what applies to your property before quoting.
How long does an extension take to build?
A single-storey extension typically takes 10 to 16 weeks on site, and a double-storey extension 16 to 24 weeks, from groundworks to decoration. Bad weather, deep foundations, steel lead times and changes to the specification can extend this.
Allow time before work starts too. Drawings usually take a few weeks, councils typically aim to decide householder planning applications within about eight weeks, and building control approval or a building warrant can run alongside or after planning. A realistic overall timeline from first survey to finished room is often six to nine months.
During the build, most of the noisy work happens outside. The disruptive stage inside is when the extension is opened up to the house, usually near the end, once the new structure is weathertight.
Is VAT charged on a house extension?
Yes. Building work to extend or improve an existing home is charged at the standard VAT rate of 20%, and all our guide prices include it. HMRC has a few specific exceptions, such as certain work to adapt a home for a disabled person, or converting a building into a different number of homes, but an ordinary extension to a house is charged at the standard rate.
When comparing prices, check whether each one includes VAT. A price that looks lower may simply leave it out, and on a large extension that difference is substantial.
How can you keep the cost of an extension under control?
The surest way to control cost is to settle the design and specification before the price is fixed, then avoid changes once work starts. A few choices make a real difference:
- Keep the footprint a simple rectangle and the roof a simple shape.
- Use standard window and door sizes where you can.
- Plan the layout around existing drains rather than moving them.
- Choose your kitchen, flooring and glazing early so they are priced properly.
- Agree any changes in writing, with the price, before the work is done.
Every quote we give follows a free site survey and sets out the agreed specification in writing. Compare typical costs on our prices page or read more about single-storey extensions.
Related services
- Single-storey extensions: guide £35,000–£75,000 inc VAT
- Double-storey extensions: guide £60,000–£120,000 inc VAT
Figures in this guide are typical UK ranges including VAT where it applies. They are not quotes. Rules on planning and building control differ between England, Wales, Scotland and Northern Ireland and change over time, so we check what applies to your property before quoting.