Big projects
House extension cost guide
An extension is the largest, most expensive and most regulated project a homeowner takes on. It cannot be priced from a form, but you can run it smoothly by understanding the stages. Even if you never lay a brick, being an informed client saves money and stress.
Updated November 2026. Estimates based on typical UK prices.
Typical UK ranges reported by the industry
These are widely reported industry ranges, not a quote. They move with material and labour costs and vary by region. A 20 m² extension at basic spec could very roughly run £36,000–£44,000. The same at high spec could be £70,000 or more. Get a quantity surveyor or 2–3 builder quotes before budgeting against any figure here.
The price
What affects the cost?
- 1
Size and number of storeys
Cost per square metre is the headline figure. Two-storey extensions share foundations and roof, so cost less per m².
- 2
Specification
Glazing, kitchen, flooring and heating dominate the budget, far more than the shell.
- 3
Ground conditions
Clay, trees and old drains all affect foundation depth and cost.
- 4
Planning and regulations
Design fees, structural calculations, building control and party wall costs add up before you start building.
- 5
Structural work
Knocking through to the existing house needs steel beams and temporary support designed by an engineer.
Be honest with yourself
Can you do this yourself?
You can do
- Acting as your own project manager
- Finishing work: decorating, flooring, tiling and fitting out once the shell is complete
- Clearing the site and managing the garden around it
Leave to a pro
- Design and structural engineering
- Foundations, brickwork, roof and structural openings
- Electrics, plumbing and heating (certified)
- Building control sign-off
The rules
Many single-storey extensions fall under permitted development, but there are limits on depth, height and boundary distance, and conservation areas and listed buildings have extra rules. Building regulations approval is needed in virtually every case, and a party wall agreement is often needed near a boundary.
What to expect
How an extension project runs, step by step
You will not do these yourself, but knowing the order lets you plan money, time and decisions.
- 1
Decide what you need and set a realistic budget
Write down the extra space you want and what it must do. Add a contingency of 10–15% to your budget, because extensions nearly always uncover surprises.
- 2
Check planning and permitted development
Ask your local authority or use the planning portal to see whether your design fits permitted development. If not, you will need a planning application, which takes weeks.
- 3
Appoint a designer and structural engineer
An architect or architectural technologist draws the plans and an engineer designs the foundations and any steelwork. These drawings are what builders quote against.
- 4
Building control and party wall notices
Submit plans to building control (or an approved inspector) and, if you are building near a boundary, serve party wall notices on neighbours.
- 5
Get at least three detailed quotes
Send the same drawings and specification to each builder, ask for itemised quotes, check references and see previous work.
- 6
Build: groundwork, shell, roof, first fix, plastering, second fix
The work follows the same order as any build: groundworks and foundations, walls and roof, first fix for pipes and cables, plastering, then second fix, fitting out and decorating.
- 7
Inspections and sign-off
Building control inspects at set stages. At the end you receive a completion certificate, which you will need when you sell. Keep all electrical and gas certificates too.
Learn from others
Common mistakes to avoid
- Starting without a written contract and a clear specification
- Not budgeting for design fees, regulations and VAT
- Paying too much up front, before the work has been done
- Skipping building control sign-off
- Having no contingency fund for surprises
Frequently asked questions
How much does a house extension cost in the UK?
Industry-reported ranges typically run from around £1,800 per m² for a basic single-storey extension to £3,500+ per m² for a high-spec one, so a modest 20 m² extension might range from roughly £36,000 to £70,000 depending on spec. These figures move with material and labour costs and vary by region, so treat them as a starting point, not a quote.
Can I build my own extension?
You can act as your own project manager and do some of the finishing work yourself, but the structural build (foundations, brickwork, roofing) needs qualified tradespeople and sign-off from building control. This isn’t a weekend DIY job in the way decorating or fencing is.
Do I need planning permission for an extension?
Often not, if it falls within permitted development rights, but there are limits on size, height and proximity to boundaries that vary by property type. Check with your local planning authority before assuming you’re covered.
How do I get an accurate price for my extension?
Get a quantity surveyor or architect to draw up a proper specification, then take that to 2 to 3 local builders for competing quotes. A detailed spec is what turns a rough per-m² range into a number you can actually budget against.
How long does an extension take to build?
Allow several months from first design to completion: design and approvals often take longer than the build itself. The build of a single-storey extension typically runs for a few months once on site.
Ready to plan it?
Add this job to your renovation plan to get the order of work, who to book and a budget you can track.