Double-storey extensions: Costs & Planning

What is a double-storey extension?

A double-storey extension adds new rooms on both the ground and first floor in one build, typically a larger kitchen or living space downstairs and an extra bedroom or bathroom above. Because the roof and foundations are shared, the cost per square metre is usually lower than a single-storey extension.

Detached brick house with a new two-storey side extension and matching hipped roof

Two-storey work has to sit comfortably with the house and the neighbours, so most of the design effort goes into the roofline, window positions and how the new first floor connects to the landing. We handle the planning application, the structural design and the building control process, then build, roof and finish both floors.

Typical time on site: 14 to 24 weeks
Guide price: £2,400 to £3,850 per m²
Building regulations: Required

What is included?

Measured survey, design and planning drawings
Householder planning application
Structural engineer’s design and calculations
Foundations, steelwork, brick and block walls, roof
Windows, insulation, plastering, electrics and plumbing
Joining the new first floor to the existing landing

How much does a double-storey extension cost?

A two-storey side extension adding about 40 m² across both floors typically costs around £115,000 to £135,000 including VAT at a good specification.

Double-storey extensions	Guide price incl. VAT
Standard specification	£2,400 to £2,900 per m²
Good specification	£2,900 to £3,400 per m²
High specification	£3,400 to £3,850 per m²

Unit: per m² of new floor area. Guide ranges only; your price is fixed in writing after the survey.

Use the guide price tool for a figure based on your size and specification, or see all guide prices.

Does a double-storey extension need planning permission?

England

Two-storey rear extensions can sometimes be permitted development in England (no more than 3 m deep, at least 7 m from the rear boundary, roof pitch to match), but most two-storey and all two-storey side extensions need householder planning permission. Expect around eight weeks for a decision once the application is validated.

Wales

Most two-storey extensions in Wales need householder planning permission from the council. We prepare and submit the application and deal with any questions from the planning officer.

Scotland

Two-storey extensions in Scotland almost always need planning permission, and a building warrant must be granted before work starts.

Northern Ireland

Two-storey extensions in Northern Ireland normally need planning permission from the council, plus building control approval.

What about building regulations?

Full building regulations approval is needed, including fire safety for the new first-floor rooms and escape routes.

What should I think about before starting?

Side extensions close to a boundary are often set back or stepped down so they do not create a terraced effect.
Neighbours’ light is assessed using the 45-degree guide many councils use.
The new roof is tied into the old one, so roof tiles and ridge details need to match.
The first-floor link can mean moving a bathroom or reshaping the landing.

How does the project run?

Tell us about the project. Call, use the guide price tool or send the form with a few photos and a postcode.
Free site visit. We measure up, look at access, drains and the structure, and talk through options and budget.
Design and fixed price. You get drawings and a written price for the agreed specification before anything is ordered.
Build and hand over. Our teams build to the programme, building control inspects, and you get the completion paperwork at the end.

Questions about double-storey extensions

How much does a two-storey extension cost?

Typically about £2,400 to £3,850 per square metre of new floor area including VAT. A 40 m² extension across two floors is often around £100,000 to £150,000 depending on specification.

Does a two-storey extension need planning permission?

Usually yes. In England some two-storey rear extensions are permitted development, but side extensions over one storey always need planning permission.

Is a two-storey extension better value than a single-storey one?

Per square metre, usually yes, because the foundations and roof serve two floors of space.

How long does a two-storey extension take?

Around 14 to 24 weeks on site, plus the planning period before that.

Will we need to move out?

Most people stay. The noisiest stage is when we break through into the existing first floor, which we plan for a short window.

Related projects

Guides

Where do we build double-storey extensions?

We take on double-storey extensions across England, Wales, mainland Scotland and Northern Ireland. Choose your area: