If you are asking how much does a house extension cost, the honest answer is usually £1,800 to £3,000 per m² for a standard single-storey build in 2026.
That puts a 20m² extension at roughly £36,000 to £60,000 for the building work and standard finishes.
A more realistic total budget may be higher once you add design fees, surveys, planning or building control, kitchen fittings, premium glazing, VAT and a contingency fund.
These are working ranges, not promises. Your property, location and specification will decide the final figure.
How much does a house extension cost per square metre?
A typical UK extension costs around £1,800 to £3,000 per m² for a standard single-storey build. Double-storey work may cost less per square metre, but the total project price is higher because you are creating more space.
The lower end may cover a simple shape, straightforward ground conditions and standard materials.
The upper end is more likely where you have:
- Large areas of glazing
- A kitchen-diner fit-out
- Difficult foundations
- Structural alterations
- Premium flooring and fittings
- A complex roof
- Higher labour costs in London or the South East
A recent UK cost guide based on homeowner quotes puts the average extension at around £52,000, but averages can hide a lot. A small side return and a large wrap-around extension should not be compared as if they are the same job.
What does a 20m² single-storey extension cost?
A 20m² single-storey extension commonly falls between £36,000 and £60,000 for construction and standard finishes. Once professional fees, approvals, VAT, kitchen work and contingency are added, the overall budget may be considerably higher.
A 10m² extension might sit around £18,000 to £30,000.
A 15m² extension might cost approximately £27,000 to £45,000.
A 20m² extension might cost approximately £36,000 to £60,000.
These examples are useful for early planning, but they are not quotes. They assume a fairly normal project with no major surprises underground and no unusually high-end specification.
Is a double-storey extension cheaper per square metre?
Usually, yes. A double-storey extension can cost less per square metre because foundations, scaffolding and parts of the roof are spread across two floors. However, the complete project normally costs more because it creates substantially more new space.
A broad 2026 guide range for a standard double-storey extension is around £1,500 to £2,500 per m².
For example, a 40m² total extension across two floors could cost around £60,000 to £100,000 before some professional fees, VAT and high-spec extras.
The exact calculation depends on whether the quoted size means the footprint or the combined floor area. Ask for this to be stated clearly. Otherwise, two builders may appear to be offering very different prices when they are measuring the job differently.
Does a rear, side or wrap-around extension cost more?
A rear extension is often the simplest option, while side return and wrap-around designs can cost more when they involve awkward access, drainage changes or complicated connections to the existing house. The shape matters, not just the floor area.
A side return may be smaller, but it can require work close to boundaries and neighbouring properties.
A wrap-around extension combines rear and side work. It creates more space but usually involves more foundations, roof junctions, structural openings and internal changes.
A simple rectangular extension is generally easier to price and build than an L-shaped design with several roof levels and large openings.
How much does a kitchen-diner extension cost?
A kitchen-diner extension costs more than a shell-only build because the finished room needs plumbing, electrics, heating, lighting, flooring, units, worktops, appliances and decoration. The kitchen itself can add tens of thousands of pounds.
A 20m² extension may cost £36,000 to £60,000 as a standard building project.
Adding a kitchen-diner fit-out can push the complete budget towards £60,000 to £100,000 or more, depending on the units, appliances, worktops, glazing and layout.
Decide early whether your budget includes the kitchen. Some builders quote only the shell and first-fix services. Others include a complete kitchen installation. The difference must be written down before you compare prices.
What is normally included in an extension quote?
A proper quote should explain whether it includes the groundworks, foundations, structure, roof, insulation, windows, external doors, plumbing, electrics, plastering, flooring, decorating and waste removal. Never assume every visible part of the finished room is included.
A standard construction quote may include:
- Site preparation
- Foundations and groundworks
- External walls
- Roof structure and covering
- Insulation
- Windows and external doors
- Basic plumbing and electrics
- Plastering
- Standard internal finishes
It may exclude:
- Architect or designer fees
- Structural engineer calculations
- Planning applications
- Building control
- Party wall surveyors
- Kitchen units and appliances
- Bi-fold doors or premium glazing
- Drain relocation
- Landscaping and patio work
- VAT
- Temporary accommodation
Ask every builder to separate included, excluded and provisional costs. That makes quotes much easier to compare.
What makes a house extension more expensive?
The biggest cost drivers are size, access, ground conditions, structural work, glazing, roof design, internal fittings and location. A survey and proper specification reveal these issues earlier, when changes are still cheaper to make.
Foundations and ground conditions
Poor ground, tree roots, sloping sites and nearby drains can increase excavation and concrete costs. You may need deeper foundations or extra drainage work.
Structural openings
Removing a rear wall or opening up the existing house may require steel beams and temporary support. The price depends on the span and the structure above it.
Glazing
Bi-fold doors, sliding doors, roof lanterns and large windows can quickly move a project towards the higher end of the budget.
Roof type
A basic flat roof may be cheaper than a pitched roof, but roof shape, insulation, rooflights and the connection to the existing house all affect the price.
Location
Labour and overheads are often higher in London and the South East. Local material availability and site access also affect the quote.
Internal specification
A shell is one thing. A finished kitchen, utility room, bathroom, underfloor heating and premium flooring are another. Specify the room use before asking for a fixed price.
Do you need planning permission for a house extension?
You may need planning permission for an extension, although some projects can fall under permitted development rights. The limits depend on the size, height, position, property type and location, so check your council before starting work.
The UK government says homeowners will probably need planning permission to make a major change to a building, such as building an extension. You can check the official planning guidance and contact your local planning authority.
Permitted development is not a blanket exemption. Restrictions may apply if your property is:
- In a conservation area
- A listed building
- In a national park or similar protected area
- Covered by an Article 4 direction
- Subject to previous planning conditions
Even where planning permission is not required, building regulations usually still apply. You may also want a lawful development certificate as written evidence that the work was permitted.
Why do you need a survey and proper drawings?
An online calculator can multiply floor area by a price band, but it cannot see your foundations, drains, wall structure, access problems or planning restrictions. A measured survey and clear drawings give builders the same information to price accurately.
You may need:
- A measured building survey
- Existing and proposed drawings
- Structural engineer calculations
- Drainage information
- Soil or ground investigation
- Planning advice
- Building regulations drawings
- Party wall advice
You do not always need a traditional architect. An architectural technologist or experienced designer may be suitable for some projects. What matters is having accurate, scaled information that a builder can price and build from.
How much contingency should you budget for?
Set aside around 10% to 15% of the build cost as a contingency. This is not money to spend on upgrades; it is protection against hidden ground conditions, design changes, delays and unexpected repairs.
For a £50,000 build, that means roughly £5,000 to £7,500.
You should also confirm whether VAT is included. Most standard building work is subject to VAT when carried out by a VAT-registered business, although specific rules and exceptions can apply.
Do not use every pound of your budget on the initial quote. A project that looks affordable on paper can become stressful if there is no money left for problems.
How should you compare house extension builders?
Compare written, itemised quotes from at least three suitable builders. Check what each price includes, ask about similar completed projects and confirm who handles building control, waste, inspections and subcontractors.
Look for a builder who answers practical questions clearly.
Ask:
- Is the price fixed or an estimate?
- What could cause it to change?
- Are materials and labour included?
- Who arranges building control?
- Are windows and doors included?
- Is the kitchen included?
- What is the expected programme?
- What insurance do you carry?
- Can I speak to recent customers?
A clear quote is not always the cheapest quote. It is the one that gives you the clearest view of what you will actually pay.
What should extension builders explain on their website?
Extension builders who answer cost questions early often win trust before the homeowner is ready to request a quote. Explain typical project bands, what changes the price, what you include and what needs a site visit.
Do not promise an exact price from a short form. Explain that a rough range is useful for planning, but a proper quote needs measurements and a specification.
If you run an extension business, check whether your website is visible when local homeowners search for extension costs and nearby builders. Request a VU1 audit to see what is helping or holding back your local visibility. It takes under 15 seconds to request — the report lands within a few minutes.
A VU1 membership costs £99/month and helps trade businesses track local visibility and turn more website visits into enquiries. See VU1 membership options.
What is the straight answer on house extension cost?
For 2026, allow roughly £1,800 to £3,000 per m² for a standard single-storey UK extension, then add professional fees, approvals, VAT, fit-out costs and a 10% to 15% contingency. Get a survey and itemised quote before committing.
The headline figure is useful, but it is only the starting point. The best budget comes from a clear design, realistic specification and builder who explains exactly what is included.