"How much will it cost?" is the first question almost every homeowner asks, and the honest answer is: it depends on size, structure, specification and location. This guide gives realistic UK ranges so you can set a sensible budget before you speak to anyone.
All figures below are typical ranges for building work only and are guides, not quotes. Prices vary by region, site access, ground conditions and the finish you choose, and they change over time, so always get a written, itemised quotation for your own project.
The short answer
As a rule of thumb, a typical single-storey or two-storey extension in the UK costs roughly £1,500 to £2,500 per square metre to build, before fit-out extras such as a kitchen. That works out at:
- Small single-storey extension (15–20 m²): about £25,000–£40,000
- Medium extension (25–40 m²): about £40,000–£90,000
- Large or complex two-storey extension: £70,000 and above
Higher-specification finishes, tricky sites and heavy structural work all push towards the top of these ranges or beyond.
What affects the price most
Size and number of storeys
Floor area is the biggest driver. A two-storey extension costs more in total than a single-storey one, but often less per square metre of usable space because the foundations and roof are shared across two floors. See our comparison of single-storey and double-storey extensions.
Foundations and ground conditions
Clay soil, nearby trees and sloping ground can mean deeper or specialist foundations. A site investigation or the structural engineer's recommendations will tell you early on.
Structural work
Opening up the back of the house usually means a steel beam. Taking out a supporting wall adds engineering, steelwork and making-good costs. We cover this in removing a load-bearing wall.
Specification
Large glazed doors, rooflights, underfloor heating, bespoke joinery and premium bricks all add up. Choosing where to spend and where to save is the most effective way to control the final figure.
Kitchens and bathrooms
If your extension includes a kitchen or bathroom, the fit-out is often priced separately. Kitchens can range from a few thousand pounds to well beyond £25,000. Read our kitchen extension guide for typical costs.
Costs people forget to budget for
Build cost is rarely the whole story. Allow for:
- Design and drawings: architectural or technical design fees, often a percentage of build cost or a fixed fee
- Structural engineer: calculations for beams and foundations
- Planning and building regulations fees: see planning permission and building regulations
- Party wall surveyor fees, if you share a wall or build close to a neighbour's boundary: see party wall agreements
- VAT: building work on an existing home is generally charged at the standard rate, so check whether quotes include it
- Temporary kitchen, storage or accommodation during the work
- Garden reinstatement, decorating, flooring and furniture
- A contingency of 10–15% for the unexpected, especially on older properties
How to keep the cost under control
- Fix your brief early. Changes mid-build are the most common cause of overspend.
- Get the design right on paper. Detailed drawings and a clear specification let builders price like for like.
- Compare itemised quotes. A very low price often means something has been left out.
- Agree provisional sums. Items you have not chosen yet, such as tiles or fittings, should be listed with an allowance.
- Keep a contingency. Treat it as part of the budget, not a bonus.
Is it worth the money?
An extension can be cheaper and less disruptive than moving once you factor in stamp duty, agents' fees and removal costs. A well-designed extension in the right area may also add value to your home, although the uplift depends heavily on the local market, so ask a local estate agent for a view before committing.
Get a realistic price for your project
Every house is different. Abron Contracting is based in Newport and can talk through your plans, explain what is likely to affect your budget, and provide a clear written quotation. Contact us or call 01633379170 to start the conversation.
Frequently asked questions
How much does a single-storey extension cost?
A typical single-storey extension costs roughly £1,500–£2,500 per square metre for the build, so a small 15–20 m² extension is often in the region of £25,000–£40,000 before fit-out and professional fees. Your actual price depends on specification and site conditions.
Is a two-storey extension cheaper per square metre?
Often, yes. The foundations and roof are shared between two floors, so the cost per square metre of floor space is usually lower, although the total cost is higher.
Do extension prices include VAT?
Not always. Check whether a quotation is stated with or without VAT. Building work on an existing home is generally standard-rated, but VAT rules can be complex, so take advice for your circumstances.
How much contingency should I allow?
Most advisers suggest 10–15% of the build cost, and more for older or unusual properties where hidden problems are more likely.
Can I get a fixed price?
A fixed price is possible once the design, structure and specification are finalised. Items still to be chosen are normally covered by provisional sums, which can go up or down.
