In short: A single storey extension costs £1,400–£2,600 per m² in London and the South East, depending on size (small £1,500–£2,500, medium £1,400–£2,350, large £1,500–£2,600 per m²). A typical 32 m² rear extension lands between £45,000 and £75,000 for the build itself.
These are Builder Expert's own figures, taken from real estimates. They already include labour, materials, the builder's margin and 20% VAT, so nothing needs adding on top. Outside London and the South East the same job costs less; see the regional table below. Figures last reviewed October 2026.
Single storey extension cost per m² in the UK: the short answer
A single storey extension costs between £1,400 and £2,600 per m² in London and the South East, depending on which size band it falls in: small £1,500–£2,500, medium £1,400–£2,350, large £1,500–£2,600 per m². The rate does not simply fall as the job grows: small extensions carry their fixed costs over very little floor area, while large ones tend to add more steel and glazing per m², so both bands reach higher rates than a straightforward 32 m² build.
These are Builder Expert's own prices, built from real estimates, not a market guess. They are what the customer actually pays: labour, materials, the builder's margin and 20% VAT are already inside the figure, so a builder or homeowner reading a quote against this page should never add VAT, overheads or profit on top.
Every region outside London and the South East costs less for the same job; the full breakdown is in the regional table further down. Wider construction costs are still moving: BCIS forecasts UK building costs to rise over the five years to 2031, which is why any per-m² figure needs a review date against it.
What our figures include (and what is not in them)
Builder Expert's prices are the whole job, start to finish: preliminaries, demolition, groundwork, structure, roof, first fix, plastering, decorating and fitting out, everything needed to hand back a finished, working extension. Labour, materials, our margin and 20% VAT are already built in, so these are the figures a customer pays, not a trade or cost price.
Three things are deliberately not inside them. There is no contingency in our figures, a sensible allowance (commonly 10% in the market) is the reader's own addition, kept separate rather than hidden in the price. Professional and statutory fees, architect, structural engineer, building control, planning, party wall, sit outside the build price; see the fees section below.
And the cost of the provisional items themselves, windows, bi-folding doors, kitchen units, sanitaryware, tiles, is a budget the homeowner sets, not a price we fix. Our bands carry an allowance for fitting them and a starting budget for the glazing, but the final figure moves with what the homeowner chooses; see the section on that further down, because it's the part that catches most people out.
Single storey extension cost by size
Builder Expert prices single storey extensions in three bands. These are the only totals on this page, and every other figure here is built from them.
| Size | Total cost (London & SE) | Usable floor area | Cost per m² |
|---|---|---|---|
| Small | £30,000–£50,000 | about 20 m² | £1,500–£2,500 |
| Medium | £45,000–£75,000 | about 32 m² | £1,400–£2,350 |
| Large | £75,000–£130,000 | about 50 m² | £1,500–£2,600 |
Notice the per-m² rate doesn't fall in a straight line as the job gets bigger, a large extension often carries more structural steel and glazing per m² than a small one, which pulls its rate back up even as the fixed costs (like a skip or a boiler) are spread over more floor area.
Single storey extension cost by region
London and the South East carry the UK's highest labour and site costs, so every other region prices lower for the same scope of work. These figures scale down from the London and South East bands above in direct proportion to regional build-cost differences.
| Region | Small (~20 m²) | Medium (~32 m²) | Large (~50 m²) |
|---|---|---|---|
| London & South East | £30,000–£50,000 | £45,000–£75,000 | £75,000–£130,000 |
| East of England | £27,500–£46,000 | £41,500–£69,000 | £69,000–£119,500 |
| South West | £26,500–£44,000 | £39,500–£66,000 | £66,000–£114,500 |
| Midlands | £25,500–£42,500 | £38,500–£64,000 | £64,000–£110,500 |
| Yorkshire & the Humber | £25,000–£41,500 | £37,500–£62,500 | £62,500–£108,000 |
| North West | £25,000–£41,500 | £37,500–£62,500 | £62,500–£108,000 |
| Wales | £24,500–£41,000 | £37,000–£61,500 | £61,500–£106,500 |
| Scotland | £25,500–£42,500 | £38,500–£64,000 | £64,000–£110,500 |
| North East | £24,000–£40,000 | £36,000–£60,000 | £60,000–£104,000 |
| Northern Ireland | £22,500–£37,500 | £34,000–£56,500 | £56,500–£97,500 |
Cost per square foot
Converting our overall range gives roughly £130–£242 per sq ft for a single storey extension in London and the South East (£1,400–£2,600 per m² ÷ 10.764 sq ft per m², spanning the lowest and highest per-m² figures across all three size bands, £1,400 from the medium band and £2,600 from the large band). As with the per-m² figures, that's the all-in price, labour, materials, margin and VAT included, and other regions sit below it.
Where the money goes: the cost split
Spread across a typical job, the build splits roughly like this. The percentages are rounded bands that sum to 100%; the pounds alongside them are illustrative only, worked from a £60,000 total, the midpoint of our £45,000–£75,000 medium (32 m²) band, so you can see where the money actually goes.
| Phase group | Share of total | Illustrative (£60,000 job) |
|---|---|---|
| Preliminaries | 12% | £7,200 |
| Demolition & groundwork | 10% | £6,000 |
| Structural shell (flooring, steel, walls, roof) | 33% | £19,800 |
| First-fix trades (joinery, plumbing, electrics) | 17% | £10,200 |
| Plastering & decorating | 9% | £5,400 |
| Finishes & fitting out | 11% | £6,600 |
| Glazing and external doors (allowance) | 8% | £4,800 |
The glazing line is an allowance only: swap standard windows for a wide bi-folding set and that share rises, pushing the whole job up its band. Preliminaries, scaffold, welfare, skips, site protection, parking and the final clean, are 10–15% of the total build cost, and they sit inside the figures quoted above, not on top of them. For the full detail on what preliminaries cover and why they vary, see our guide to preliminaries in construction.
What the work involves, phase by phase
Builder Expert prices every extension in the same build order. Walking through it shows what's actually driving the cost at each stage:
- Preliminaries, scaffold, welfare, site security, skip hire and protection for the duration of the job.
- Demolition, removing the existing external wall, windows, flooring and anything else in the footprint, and clearing it from site.
- Foundations and footings, excavating and casting trench foundations, footings up to DPC, and reinforced ground beams where a large opening needs extra support.
- Underground drainage, diverting or extending drainage runs, manholes, gullies and soakaways that the new footprint disturbs.
- Structural flooring, the new slab with damp-proof membrane and insulated screed.
- Structural and steelwork, beams, columns and connections that carry the new opening and roof loads.
- External walls, brick and block cavity walls, new openings, lintels and fire barriers.
- Roofing, the new roof structure, insulation, covering, fascia, soffits and rainwater goods.
- Joinery, internal stud walls, doors, skirting and architraves.
- Plumbing and heating, radiators, pipework, and any new boiler, cylinder or underfloor heating the extension needs.
- Electrics, sockets, lighting, ventilation, alarms and any extra fuse board capacity.
- Plastering, plasterboard and skim to every new wall and ceiling.
- Decorating, preparation and finishing throughout the new space.
- Finishes, floor and wall tiling, flooring and splashbacks.
- Fitting out, sanitaryware, kitchen units, worktops and appliances.
- Provisional sums, the homeowner's own budgets for windows, doors and fitted items, covered next.
How to check a builder's per-m² figure against the measured work
A useful sanity check on any quote: an extension only adds three new walls, the two side walls plus the new rear wall (width + twice the length), because the fourth side already exists as the house. Roof area is measured as footprint × 1.1 to allow for overhangs. If a quote's wall or roof quantities look far larger than that, ask what's being priced.
Windows, bi-folds and kitchens: the budgets you set, not the builder's price
Some items in an extension aren't priced by the builder at all, they're a budget the homeowner sets, and the builder fits what's bought. External doors and windows, bi-folding or sliding doors, kitchen units, worktops, sanitaryware, floor and wall tiles and ironmongery all work this way. Builder Expert's figures cover the labour to fit them; the item itself is the customer's choice and cost.
The catch is that the builder's margin and VAT still apply on top of that budget, so a provisional sum costs roughly 1.44 times its face value by the time it reaches the final bill. A £7,000 kitchen allowance, for example, ends up costing around £10,080 once margin and VAT are added.
As a guide to current UK market pricing for the two biggest provisional items on most extensions:
| Item | Typical UK market budget | Notes |
|---|---|---|
| Bi-folding or sliding doors | £2,000–£6,000 per set (supply only) | A typical 3–4 m set; wider spans, slimmer aluminium frames and triple glazing cost more; add margin and VAT on top |
| Windows and external doors | £400–£1,200 per window; £800–£2,000 per door (supply only) | uPVC at the lower end, aluminium and timber higher; add margin and VAT on top |
These are current market averages, not Builder Expert prices, treat them as a starting budget and expect the final figure, once margin and VAT are added, to land noticeably higher.
Fees and statutory costs on top of the build
These are market prices, separate from Builder Expert's build figures above:
| Fee | Typical cost | Notes |
|---|---|---|
| Architect / design fees | Typically 5–12% of build cost | Lower for drawings only, higher for full project management |
| Structural engineer fees | Typically £500–£1,500 | Calculations for beams and foundations on a standard extension |
| Building control application | Typically £600–£1,200 | Council or approved inspector, including inspections through the build |
| Householder planning application (England) | £528 | Since 1 April 2025; set by the Planning Portal fee schedule |
| Lawful development certificate (proposed work) | £264 | Half the planning fee |
| Party wall surveyor | Market range, typically £700–£2,000+ per surveyor | Needed where work is near a shared boundary |
Worked example: a 32 m² rear extension
Take a 32 m² rear extension with one goalpost opening onto a kitchen-diner, a typical medium job. In London and the South East, Builder Expert's own figures put the whole build, start to finish, at £45,000–£75,000, or £1,400–£2,350 per m². That includes labour, materials, margin and VAT; it does not include a contingency, professional fees, or the homeowner's own budget for windows, bi-folds and the kitchen.
Take the midpoint of that band, £60,000, as this job's London and South East figure. The same extension in the North East scales down by that region's factor to about £48,000, roughly 20% less for the identical scope, reflecting lower regional labour and site costs rather than a cheaper specification, and sitting comfortably inside the region's £36,000–£60,000 medium band. If you want to see how a figure like this is set out phase by phase before you commit to anything, our worked estimate example shows what a priced estimate actually looks like.
Neither figure includes a contingency. No contingency is built into Builder Expert's figures, a common market practice is to set your own allowance, typically around 10% of the build cost, and keep it separate rather than assume it's already covered.
What pushes a single storey extension to the top of the range
Several things push the same size of job from the bottom to the top of its band:
- Ground conditions and foundation depth, poor or variable ground needs deeper or wider foundations
- Restricted access or hand-digging, a terraced property with no side access costs more to excavate and build
- Large structural openings, each large opening is a goalpost: two beams, two columns and four connections, rather than a single beam
- Roof type, a flat roof is cheaper than a pitched roof matching the existing house
- Drainage diversions and manholes, rerouting existing drainage runs adds cost that a clear run avoids
- Glazing and finish level, full-width sliding doors and a high-spec finish sit well above a standard window and painted plaster
How long a single storey extension takes
As a typical programme rather than a guarantee, site time on a single storey extension runs roughly:
| Size | Typical time on site |
|---|---|
| Small (~20 m²) | 8–10 weeks |
| Medium (~32 m²) | 10–14 weeks |
| Large (~50 m²) | 14–20 weeks |
Design, planning and building control sign-off happen before any of this, and usually add several more months ahead of the build itself.
Is a 3 metre extension worth it, and should you extend or move?
How much does it cost to extend a house in the UK? As set out above, Builder Expert's own figures put a single storey extension at £30,000–£130,000 depending on size, with London and the South East at the top and other regions lower.
Is a 3 metre extension worth it? A 3 m rear extension is usually the smallest size that meaningfully improves a kitchen-diner, and it's also the depth most terraced and semi-detached homes can build under Permitted Development without planning permission, which keeps both cost and timeline down. It's worth it where the extra space solves a genuine layout problem; it's a poor spend where the room was already big enough and the money is better put toward finish quality.
What is the '50% rule' for extensions? Under Permitted Development, a house cannot have additions, extensions, outbuildings and other structures combined, that cover more than 50% of the land around the "original house" (the property as it stood in 1948, or as first built if later). Go over that and full planning permission is needed regardless of the extension's size. The limits are set out in the government's permitted development rights for householders technical guidance.
Should you extend or move? Moving costs money too, stamp duty, agent fees, legal fees and the stress of a chain, and an extension keeps you in a known location and school catchment. As a rule of thumb, if an extension's cost (from the tables above) is less than the stamp duty and fees of moving to a bigger house, and the extended layout solves the problem, extending usually wins. Our guide on how to price an extension covers the method a builder uses to get from this per-m² guide to a firm figure.
Single storey extension cost FAQs
How much does a single storey extension cost per m² in the UK?
£1,400–£2,600 per m² in London and the South East across our size bands (small £1,500–£2,500, medium £1,400–£2,350, large £1,500–£2,600), Builder Expert's own figures including labour, materials, margin and VAT; less elsewhere, see the regional table above.
What does a small, medium and large extension cost in total?
£30,000–£50,000 for about 20 m², £45,000–£75,000 for about 32 m², and £75,000–£130,000 for about 50 m², all London and South East prices.
What is included in the price and what is not?
Labour, materials, margin and VAT are included. Contingency, professional fees and provisional sums (windows, bi-folds, kitchen) are not.
How do provisional sums affect the total?
They're a homeowner's own budget, not our price, but margin and VAT still apply on top, at roughly 1.44 times the budget value.
What fees sit on top of the build?
Architect and engineer fees, building control, a £528 planning application (England), a £264 lawful development certificate, and party wall surveyor fees where they apply.
Why do two quotes for the same extension differ so much?
Usually ground conditions, access, the number of structural openings, roof type, drainage work and glazing level, see the section above on what pushes cost to the top of the range.
Getting a measured price for your extension
A per-m² guide is a starting point, not a quotation, the only way to know what your extension actually costs is to measure it against your own drawings. Builder Expert's construction estimating services turn a set of drawings into a priced estimate using the same rates behind every figure on this page; a basic construction estimate is the fastest route to a number you can plan around, and our construction cost calculator is a quick way to sense-check a build cost per m² before you commit to a full estimate.
These figures are indicative, current as at October 2026, and are not a quotation. Last reviewed October 2026.