House Extension Cost by Size: Every Size Priced (2026 UK)
In 2026, UK house extensions cost £1,650-£4,200 per m² depending on region and specification. That puts a 3m x 3m extension at £16,500-£38,000, a 4m x 4m at £26,500-£67,000 and a 4m x 5m at £33,000-£84,000, single storey, fully finished, including VAT. Below, every popular size from 2m x 3m to 6m x 6m is priced, single and double storey.
- ✓ 12 dimension combinations priced, 4m² to 36m²
- ✓ Per-m² rates for basic, standard and premium spec
- ✓ Double-storey costs: 1.5-1.6x single storey, not 2x
- ✓ Full 4m x 4m cost breakdown, groundworks to fees
Extension Cost by Size: Quick Answer
House extension costs in 2026 work out at £1,650-£4,200 per m²: roughly £1,650-£2,100/m² for a basic spec, £2,100-£2,800/m² for a standard spec and £2,800-£4,200/m² for premium builds and London. A typical single-storey extension starts from £22,000, with regional starting prices of £22,000-£33,500 across our 519-town dataset.
By size, that means a 3m x 3m costs £16,500-£38,000, a 3m x 4m £20,000-£50,500, a 4m x 4m £26,500-£67,000 and a 5m x 5m £41,000-£105,000, single storey. A double-storey extension starts from £44,000 (£44,000-£67,500 regionally): about 1.5-1.6 times the single-storey figure, because the roof, foundations and groundworks are shared. See our full house extension cost guide for the complete picture.
Jump to: Cost by dimensions · Cost by floor area · Cost per m² · 4m x 4m breakdown · Size comparison chart · Popular sizes explained · FAQs
House Extension Cost by Size: The Full 2026 Table
Every price below is a fully finished, all-trades cost including VAT at 20%: groundworks, shell, roof, glazing, first and second fix, plastering, decorating and professional fees. The single-storey range runs from basic spec in the cheapest regions (£1,650/m²) to premium spec in London (£4,200/m²); most projects land mid-band. Double-storey figures are the same footprint with two full floors.
| Extension size | Floor area | Single storey (basic → premium) | Double storey | Typical use |
|---|---|---|---|---|
| 2m x 2m | 4m² | £10,000-£18,000* | Rarely built | Porch, WC, small utility |
| 2m x 3m | 6m² | £13,500-£26,000* | Rarely built | Utility + WC, boot room |
| 3m x 3m | 9m² | £16,500-£38,000 | £44,000-£60,500† | Kitchen widening, home office |
| 3m x 4m | 12m² | £20,000-£50,500 | £44,000-£80,500† | Kitchen extension |
| 3m x 5m | 15m² | £25,000-£63,000 | £44,000-£101,000† | Kitchen-diner |
| 4m x 3m | 12m² | £20,000-£50,500 | £44,000-£80,500† | Dining room, playroom |
| 4m x 4m | 16m² | £26,500-£67,000 | £44,000-£107,500† | Kitchen-diner, living room |
| 4m x 5m | 20m² | £33,000-£84,000 | £49,500-£134,500 | Open-plan kitchen-diner |
| 5m x 3m | 15m² | £25,000-£63,000 | £44,000-£101,000† | Full-width shallow rear, galley kitchen |
| 5m x 5m | 25m² | £41,000-£105,000 | £62,000-£168,000 | Kitchen + family room |
| 6m x 4m | 24m² | £39,500-£101,000 | £59,500-£161,000 | Full-width rear extension |
| 6m x 6m | 36m² | £59,500-£151,000 | £89,000-£242,000 | Wraparound, large open-plan |
*Sizes under about 8m² price above the headline per-m² rates because design fees, groundworks set-up and scaffolding do not scale down with the floor area. †Double-storey projects rarely price below about £44,000 whatever the footprint, for the same reason: the scaffold, staircase alterations and structural work are broadly fixed. Prices last reviewed July 2026.
Want a number tuned to your postcode and spec rather than a national band? Our extension cost calculator does exactly that in under a minute, or you can get three fixed quotes from local builders and skip the estimating altogether.
Extension Cost by Floor Area: 10m² to 50m²
If you think in square metres rather than dimensions, use this table. The floor area is the footprint you are adding; for double storey, the same footprint gives you twice the floor space. The standard-spec column (£2,100-£2,800/m²) is where most 2026 builds outside London actually land.
| Floor area | Example size | Single storey (full range) | Single storey (standard spec) | Double storey |
|---|---|---|---|---|
| 10m² | 2.5m x 4m | £16,500-£42,000 | £21,000-£28,000 | £44,000-£67,000 |
| 15m² | 3m x 5m | £25,000-£63,000 | £31,500-£42,000 | £44,000-£101,000 |
| 20m² | 4m x 5m | £33,000-£84,000 | £42,000-£56,000 | £49,500-£134,500 |
| 25m² | 5m x 5m | £41,000-£105,000 | £52,500-£70,000 | £62,000-£168,000 |
| 30m² | 5m x 6m | £49,500-£126,000 | £63,000-£84,000 | £74,500-£201,500 |
| 40m² | 5m x 8m | £66,000-£168,000 | £84,000-£112,000 | £99,000-£269,000 |
| 50m² | 5m x 10m | £82,500-£210,000 | £105,000-£140,000 | £124,000-£336,000 |
A few worked answers to common searches: a 15 square metre extension costs £25,000-£63,000 across the full spec range, with most standard builds at £31,500-£42,000. A 20m² extension costs £33,000-£84,000 (typically £42,000-£56,000), a 30m² extension £49,500-£126,000 (typically £63,000-£84,000) and a 40m² extension £66,000-£168,000 (typically £84,000-£112,000). Above 40m² a double-storey design often makes better financial sense: see our double-storey extension cost guide.
Extension Cost per m² UK 2026: Basic, Standard and Premium
House extension cost per m² in the UK runs £1,650-£4,200 in 2026. That is a wide band, but it splits into three clean tiers, and once you know your tier the maths is simple: floor area x rate = build cost. A 12m² extension at a standard £2,450/m² is about £29,500; the same extension in premium London spec at £3,500/m² is £42,000.
| Spec tier | Cost per m² | What you get | Where it applies |
|---|---|---|---|
| Basic | £1,650-£2,100 | Blockwork shell, uPVC windows and french doors, felt or fibreglass flat roof, standard plaster and emulsion, laminate or vinyl flooring, minimal steelwork, standard strip foundations | Cheapest regions (North East, Yorkshire, Northern Ireland), simple sites, no knock-through |
| Standard | £2,100-£2,800 | Brick or rendered shell, aluminium bifolds or a large slider, one or two rooflights, warm flat roof or simple pitched roof, one steel for open plan, tiled or engineered wood floor, underfloor heating optional | Most of the UK: Midlands, North West, Wales, Scotland, much of the South outside London |
| Premium | £2,800-£4,200 | Architectural glazing, structural glass or roof lanterns, extensive steelwork, high-end kitchens-ready services, bespoke joinery, complex groundworks, zinc or standing-seam roofs | London and the South East, designer builds, difficult access or sloping sites |
Two caveats. First, below about 10m² the fixed costs (design, building control, site set-up, scaffold) push the effective rate up by 15-30%. Second, the rate covers the building work, not what goes in afterwards: a new kitchen is priced separately, and our kitchen extension cost guide covers the combined project.
4m x 4m Extension Cost Breakdown (Single Storey, 16m²)
The 4m x 4m is the most quoted extension size on BestBuilders, so here is where the money actually goes on a standard-spec single-storey build in 2026. Shares are approximate and shift with your choices: glazing is the line most people push up, groundworks the one the site conditions decide for you.
| Element | Cost (standard spec) | Share of total |
|---|---|---|
| Groundworks and foundations | £5,500-£7,500 | ~16% |
| Shell: walls, blockwork and brickwork | £7,000-£9,500 | ~20% |
| Roof (warm flat roof or simple pitch) | £4,000-£6,000 | ~12% |
| Steelwork and knock-through | £1,800-£3,200 | ~6% |
| Glazing: bifold or slider, window, rooflight | £4,500-£7,500 | ~15% |
| Electrics (first and second fix) | £1,500-£2,500 | ~5% |
| Plumbing and heating | £1,500-£3,000 | ~6% |
| Plastering, flooring and decorating | £3,000-£4,500 | ~9% |
| Professional fees, structural engineer, building regs (12-15%) | £4,000-£6,000 | ~12-15% |
| Total | £33,000-£49,500 | Typically £39,000-£42,000 |
Assumes straightforward access, no drain diversions, standard strip foundations and a single steel into the existing rear wall. Fees include architectural drawings, structural calculations and building control, at the usual 12-15% of build cost. VAT at 20% included throughout.
Single-Storey Extension Cost Compared by Size
The chart shows typical fully finished costs at a standard specification (around £2,450/m², the mid-point of the standard tier). The 4m x 4m, highlighted in orange, is the size we are asked about most. Cost rises with area, but the rate per m² quietly falls as the build gets bigger.
Popular Extension Sizes Priced and Explained
2m x 3m extension cost (6m²)
A 2m x 3m extension costs £13,500-£26,000 single storey in 2026. It is the classic utility-and-WC or boot-room addition, but poor value per square metre: at 6m² the fixed costs of any build are spread over very little floor, so the effective rate is £2,250-£4,300/m². If the budget can stretch to 3m x 3m you get 50% more room for roughly 20-30% more money.
3m x 3m extension cost (9m²)
A 3m x 3m extension costs £16,500-£38,000 single storey, and most standard-spec builds outside London land between £19,000 and £25,000. Nine square metres is enough to widen a kitchen, create a garden room or add a decent home office. At 3m deep it usually sits within permitted development on an attached house, though you should always confirm: our extension planning permission guide walks through the limits.
3m x 4m and 4m x 3m extension cost (12m²)
These are the same 12m² build in two orientations, and both cost £20,000-£50,500 single storey. The average cost of a 4m x 3m extension at standard spec is about £29,500 (12m² x £2,450), which matches what we see quoted across most of the UK; London pushes it to £35,000-£42,000. A 3m-deep, 4m-wide version is the archetypal kitchen extension; the 4m-deep, 3m-wide version suits a dining room or playroom off a side return.
4m x 4m extension cost (16m²)
A 4m x 4m extension costs £26,500-£67,000 single storey, with typical standard-spec builds at £33,500-£45,000 and a national sweet spot around £39,000. Sixteen square metres comfortably takes a kitchen-diner with an island, which is why it is the most requested size on the site; the element-by-element breakdown is in the table above. Note that 4m is the permitted-development projection limit for a detached house (3m attached) without prior approval.
4m x 5m extension cost (20m²)
A 4m x 5m extension costs £33,000-£84,000 single storey, typically £42,000-£56,000 at standard spec. Twenty square metres is proper open-plan territory: kitchen, dining and seating in one space, usually with bifolds across the garden elevation and a couple of rooflights. Glazing is the line to watch here; it can swing the total by £5,000-£10,000 on its own.
5m x 3m extension cost (15m²)
A 5m x 3m extension costs £25,000-£63,000 single storey, typically £31,500-£42,000. Wide and shallow is a clever shape: a 5m-wide, 3m-deep addition across the back of a terrace or semi transforms the ground floor while normally staying inside the 3m permitted-development projection for attached homes.
5m x 5m extension cost (25m²)
A 5m x 5m extension costs £41,000-£105,000 single storey, typically £52,500-£70,000. At 25m² you are adding a kitchen plus a family room, and at 5m deep you are beyond standard permitted development on most homes, so budget time for a planning application or the larger home extensions prior-approval route. Expect more steel, and a warm flat roof with lanterns as the common answer to the bigger span.
What Affects the Price at Any Size
Two extensions with identical dimensions can be £30,000 apart. These are the factors doing the work, roughly in order of impact.
1. Region and labour rates. London and the South East run 20-30% above the cheapest regions. The same 4m x 4m that prices at £34,000 in the North East can be £48,000 in outer London before the spec changes at all.
2. Specification and finish. The single biggest swing. Moving from basic to premium spec more than doubles the per-m² rate, and most of that difference is choices, not necessities: glazing, flooring, roof covering, joinery.
3. Glazing. Bifolds, sliders, rooflights and lanterns routinely account for 15% of the build. An aluminium slider can cost £1,500-£2,500 per linear metre installed.
4. Groundworks and site conditions. Clay soil, tree roots, sloping gardens, poor access and drain diversions all add cost before anything shows above ground. A drain diversion alone is commonly £1,500-£3,500.
5. Structural openings and steel. Every steel beam means an engineer, fabrication and a crew to set it. Fully opening the rear wall costs meaningfully more than keeping a doorway.
6. Roof type. A warm flat roof is the budget baseline; a pitched tiled roof typically adds £2,000-£5,000 at these sizes.
7. Fees and compliance. Drawings, structural calculations, building control and party wall agreements where you build near a boundary: 12-15% of build cost, and unavoidable.
How to Save Money on Your Extension
- Keep the footprint rectangular. Corners cost money; an L-shape or wraparound adds foundations, steel and roof complexity for the same floor area.
- Use off-the-shelf glazing sizes. Standard-size bifolds and rooflights can be 30-40% cheaper than made-to-measure for a near-identical look.
- Choose a warm flat roof. At small and medium sizes it saves £2,000-£5,000 over a pitched roof and takes rooflights or a lantern happily.
- Keep wet rooms near existing plumbing. A WC or utility placed against the current soil stack costs far less than one that needs new drainage runs across the slab.
- Stage the fit-out. Get the shell, glazing and plaster done, then decorate and fit flooring in phases as budget allows. The expensive trades are in the structure.
- Do not move drains, trees or boundaries if you can avoid it. Shifting the footprint half a metre at design stage is free; discovering a shared sewer under your slab is not.
- Tender it properly. Three like-for-like quotes on the same drawings is the single most reliable saving there is. Get three free quotes from vetted local builders here, and sanity-check them against our extension cost calculator first.
Regional Price Differences in 2026
Every table on this page is a national band, and your region decides where in the band you sit. Across our 519-town dataset, London and the South East price 20-30% above the cheapest regions, which are typically the North East, Yorkshire and Northern Ireland. The Midlands, North West, Wales and most of Scotland sit in the middle.
In practice: a typical single-storey extension starts from £22,000 in the cheapest areas and from around £33,500 in the dearest, and a double-storey from £44,000 up to £67,500 on the same regional spread. For a 4m x 4m at standard spec, think roughly £34,000 in the North East, £38,000-£42,000 across most of England and Wales, and £45,000-£55,000 in London. Labour is the driver; materials vary far less by postcode.
FAQs
A 3m x 3m (9m²) single-storey extension costs £16,500-£38,000 in 2026 across the full basic-to-premium range, and most standard-spec builds outside London land between £19,000 and £25,000. That is a fully finished price including groundworks, shell, roof, glazing, plastering, decorating, professional fees and VAT. In London and the South East expect 20-30% more than the cheapest regions.
A 4m x 4m (16m²) single-storey extension costs £26,500-£67,000 in 2026, with typical standard-spec builds at £33,500-£45,000 and around £39,000 as a national mid-point. The biggest cost lines are the shell (about 20%), groundworks (about 16%) and glazing (about 15%), with professional fees adding 12-15% on top of the trades. A double-storey version of the same footprint runs from about £44,000.
The average cost of a 4m x 3m (12m²) single-storey extension at a standard 2026 specification is about £29,500, based on £2,450 per square metre. The realistic range is £20,000-£50,500: basic spec in the cheapest regions sits near the bottom, premium spec in London near the top. It is the classic kitchen extension footprint, and orientation (3m deep and 4m wide, or the reverse) makes no difference to the price.
House extension costs in the UK run £1,650-£4,200 per square metre in 2026 depending on region and specification. Basic spec is roughly £1,650-£2,100/m², standard spec £2,100-£2,800/m², and premium or London builds £2,800-£4,200/m². Multiply your floor area by the rate for your tier for a quick estimate, and note that extensions under about 10m² price 15-30% above these rates because fixed costs do not scale down.
A 20m² extension (for example 4m x 5m) costs £33,000-£84,000 single storey in 2026, and most standard-spec builds come in at £42,000-£56,000. As a double-storey extension the same footprint costs £49,500-£134,500 and gives you 40m² of new floor space, which is why building up as well as out is usually the better cost per square metre.
No. A double-storey extension typically costs 1.5-1.6 times the single-storey price for the same footprint, not double, because the expensive shared elements (foundations, groundworks, roof) are built once and serve both floors. Double-storey extensions start from about £44,000 in 2026 (£44,000-£67,500 regionally). The second storey is usually the cheapest floor space you can add to a house.
Because a large share of any building project is fixed: design and structural fees, building control, site set-up, scaffold, skips and the disruption premium of getting trades to a small job. On a 4m² or 6m² extension those fixed costs are spread over very little floor area, pushing the effective rate 15-30% above the headline £1,650-£4,200/m² band. The per-m² rate falls steadily as the build gets bigger.
Yes. All prices on this page include VAT at 20% unless stated otherwise, and they are fully finished costs: groundworks, shell, roof, glazing, electrics, plumbing, plastering, decorating and professional fees. They do not include what goes in afterwards, such as a new kitchen, which is priced separately.
Allow 10-14 weeks on site for a typical 4m x 4m single-storey extension: roughly 2-3 weeks of groundworks, 3-4 weeks of shell and roof, then glazing, first and second fix, plastering and decorating. Add 6-10 weeks before that for design and building control, longer if a planning application is needed. Double-storey builds run 14-20 weeks on site.
Many single-storey extensions fall under permitted development: broadly up to 3m of rear projection on an attached house or 4m on a detached one, with up to 6m or 8m possible under the prior-approval route. Go beyond that, build higher than 4m, or add a double storey within 7m of the rear boundary and you need a planning application. Our planning permission guide covers the rules size by size.
Per square metre, yes. The fixed costs of design, fees, set-up and scaffold barely change with size, so a 25m² extension costs far less per square metre than a 9m² one at the same spec. Build the size you need, but do not shrink the plan by a metre expecting to save proportionally; the saving is always smaller than the lost space suggests.
Start with our extension cost calculator for an estimate tuned to your size, spec and region, then get three like-for-like quotes on the same drawings from local builders. Quotes are where the real number lives: national bands tell you what is reasonable, but only a builder who has seen your access, drains and ground can price your project. Request your three free quotes here.
Know Your Size? Get It Priced Properly
You now know what your extension should cost, whatever the size. The next step is three like-for-like quotes from vetted local builders: free, in about 60 seconds, no obligation.
Get My 3 Free Quotes →