Before my kitchen extension started, I wanted one number: what does a kitchen extension actually cost once every invoice lands? I wanted the real all-in figure for structure, specialist trades, glazing, kitchen fit-out, and surprises the builder's quote does not cover.
After completion in January 2024, the numbers were stark: the builder's original quote was £45,500. The final construction cost, before kitchen units and appliances, reached £122,150. Add the kitchen and the true all-in total hit £181,916. This guide explains where that extra spend comes from.
The Short Answer
A single-storey rear kitchen extension in England typically costs:
- £1,100–£1,500 per m² for the builder's structural work alone
- £2,200–£3,000 per m² all-in before kitchen units and appliances
- £2,500–£3,600 per m² all-in with a mid-to-premium kitchen package included
- £62,500–£90,000 total for a typical 25m² extension (fully kitted out)
- £100,000–£144,000 total for a 40m² extension (fully kitted out)
The reason the final all-in range is so wide is that it depends on your choices: bifold doors or standard French door, stone worktop or laminate, premium boiler or basic boiler.
My build: 55m² of new space, completed over 35 months. Construction cost (excluding kitchen) was £122,150, roughly £2,221 per m². Add the kitchen units, appliances, worktops, and fittings and the true all-in total was £181,916, roughly £3,308 per m², in the upper half of the typical range because of a premium kitchen alongside bifold doors and an unusually complicated steelwork saga.
Why the Builder's Quote Is Only Half the Story
When a builder quotes for a kitchen extension, they're pricing what they directly build: the foundations, walls, roof structure, and basic fit-out. They are not pricing:
- Architect's drawings
- Planning and building control fees
- Steelwork (often a provisional sum or separate quote)
- Windows and doors (often quoted separately or excluded entirely)
- Utility meter moves (a separate job with your energy supplier)
- Electrics (a separate trade)
- Plumbing (a separate trade)
- Screed (often a provisional sum)
- Plastering (sometimes included, sometimes not)
- Kitchen units and worktops
- Kitchen fitting labour
- Tiling
- Flooring
- Decoration
- Garden reinstatement
- Waste disposal
Every item on that list is real money. The builder's own invoices came to £53,142 against that £45,500 quote, and £7,495 of it was steel supplied through him. Even so, the builder was less than half of a £122,150 construction cost. The rest was made up entirely of the categories above, before the full kitchen package (units, appliances, worktops and fittings) added a further £59,766.
If you want the structured version of this budget rather than the blog overview, start with budgeting and contingency, getting quotes and comparing, and utility meter relocation.
Explore the full Kitchen Extension Guide
Every task, in the right order, with tools and materials guidance.
View the GuideFull Cost Breakdown: Category by Category
Here is what each category cost on my build, alongside the 2026 UK market range.
1. Professional Fees (Architect, Planning, Building Control)
You cannot start building without drawings, and you cannot submit drawings without fees.
Architect or architectural technologist: A full package for a single-storey extension (outline drawings, detailed plans, building control documents, and submission) costs £1,500–£2,500. My architect's drawing and submission packages came to £1,515, plus £315 for drawing modifications and £510 for structural calculations as the steelwork design changed. With the planning fee and OS maps paid through them, their invoices totalled £2,604.
Planning application fee (England 2026): £548 for a householder application. (This was £206 when I applied in 2021; fees have more than doubled since then.) If your extension falls under permitted development rights, you can skip planning permission, but many homeowners apply for a lawful development certificate for certainty. That costs £274 in England (half the planning fee). Our guide on whether you need planning permission covers how to confirm which route applies.
Building control full plans application: £250–£550 depending on your local authority and the size of the extension. I paid £882 for a 55.5m² extension.
Party wall: If your extension is within 3 metres of a neighbouring property's wall, you need to serve a party wall notice. If your neighbour consents, there is no cost. If they appoint a party wall surveyor, expect £700–£1,500 per neighbour. Our full guide on party wall agreements explains when the Act applies and how to avoid paying for two surveyors.
Thames Water (or your water company) build-over agreement: If you're building over or near a public sewer, you need a build-over agreement. I paid £299.
Total professional fees and council charges: £3,000–£6,000 for a typical project.
2. Builder's Structural Work
This is what the headline quote covers: digging foundations, building walls, erecting the roof structure, making the existing house weathertight, and knocking through the opening.
For a typical single-storey rear extension:
- Foundations: £200–£350 per m² of floor area (normal ground conditions)
- Walls: £7,000–£14,000 total (cavity blockwork to wall plate height)
- Roof structure: £4,000–£7,000 (including insulation, excluding covering)
- Roof covering: £2,500–£4,500 for a typical 25–30m² extension
- Scaffolding: £1,200–£2,500
Total builder's structural work: £25,000–£45,000 for a 25–40m² extension, depending on complexity, size, and location.
3. Steelwork
Almost every kitchen extension requires at least one RSJ steel beam where you knock through the existing house wall. This is typically included in the builder's quote as a provisional sum, but the actual cost varies widely with complexity.
- Simple opening (single RSJ): £900–£1,600 supply and installation
- Complex steelwork (longer spans, corner posts, bifold openings): £3,000–£6,000+
Steelwork on one documented build came to £7,495 in fabrication across six separate invoices, an outlier caused by a cantilever design, a builder who made unauthorised changes to the structural engineer's spec, and two separate steel redesigns. A straightforward opening would have cost a fraction of that.
Typical steelwork: £3,000–£8,000. Always ask the structural engineer to specify the steelwork in their scope and ensure your builder prices it as a confirmed figure, not a provisional sum.
4. Glazing (Windows, Bifolds, Roof Lights)
This is where cost ranges get enormous, driven almost entirely by what you choose.
Standard windows for a single-storey extension: £2,000–£5,000 supply and fit.
Aluminium bifold doors (3–5 panels, supply only): £3,150–£7,200. Add £600–£1,200 for installation labour.
Roof lights: £1,000–£3,000 each depending on size and specification.
My glazing was at the top end: HBD Systems roof lights (£11,748) plus Supreme Bifolds replacement doors after my original supplier went bust (£16,743). That's £28,491 before the extras: £1,812 of lead flashing, foam and fitting around the roof windows, less a £284 credit because the failed supplier's refund came back higher than I had paid. Total glazing: £30,019. For a standard single-storey with a French door and one roof light, you'd be looking at £4,000–£8,000.
Budget for glazing based on what you actually want. The gap between a basic door-and-window setup and a premium bifold-and-skylights configuration is £15,000+.
5. Utility Meter Moves
If your gas or electricity meter is on the wall where the extension is being built, it must be moved before construction starts. The supply cable or pipe is moved by your Distribution Network Operator (DNO) for electricity and your gas network operator for gas, and that is the part you pay for. The meter on the end of it is moved by your energy supplier on a separate visit. You book both and sequence them.
- Gas meter relocation: £600–£1,200 (scheduled, without complications)
- Electricity meter relocation: £500–£1,200
On one documented build the gas service alteration (SGN) was quoted at £808.80 and the electricity service alteration (SSEN) cost £1,020.70. Both figures are for moving the pipe and the cable; the suppliers moved the meters themselves without charging for it. The gas half did not run to plan. A leak was found on the day the gas trench was dug, SGN attended as an emergency, and that visit rerouted the supply to its new position, which was the scheduled job. The booking was cancelled and later refunded, and the emergency reroute was charged at £856.67 instead, so the gas network was paid once, £48 more than quoted. The reroute also happened sooner than the booked slot would have delivered it, but that upside depended on SGN already being lined up to do the work.
Beyond the meter moves themselves, the extension also needed a groundwork contractor to dig service trenches (for the electricity duct, gas main, and water main running to the new structure), install a 1m² soakaway with geotextile membrane, and fit two back-inlet gullies connecting to the sewer. This was a separate, specialist job (not the main builder's work). On the documented build it came in at £3,250, mostly in cash, paid to an independent drainage and groundwork contractor found via MyBuilder. It is easy to overlook in budgeting, and homeowners regularly get caught out by it when it isn't in the builder's quote.
Total utility costs on that build (service alterations + drainage groundwork): £5,128, made up of the gas alteration as finally charged, the electricity alteration and the drainage.
If your meters don't need moving, this category is zero. If they do, budget £1,500–£3,000 for straightforward moves, more if you need trenching work. Budget £2,500–£4,000 separately for drainage groundwork (service trenches, soakaway, gullies), almost always with a separate contractor.
6. Electrics
A kitchen extension needs substantial electrical work: first fix cabling for sockets, lighting, underfloor heating controls, and cooker supply; second fix for installing fittings; and potentially a consumer unit upgrade.
- Extension electrics (both fixes): £1,500–£5,000 for a standard specification
- Additional fittings (chrome faceplates, dimmer switches, downlights): £200–£500 on top
My electrician quoted £3,290 for the core wiring (excluding fittings, which he bought separately). By the end, including consumer unit work, the move of the unit during construction, additional jobs, and materials, I had paid £6,624 across eight invoices. Consumer unit upgrades and smart home additions (Ring doorbell, Hive thermostat, extra island sockets) drove that upward.
Typical extension electrics: £3,000–£6,000 for a well-specified kitchen.
7. Plumbing
Plumbing scope varies the most of any trade. At minimum, you need hot and cold water to the sink and dishwasher; at maximum, you're rerouting a boiler, adding underfloor heating, installing a quooker tap, and running pipes for a fridge ice maker.
- Extension plumbing (full specification, excluding boiler): £5,000–£8,000
- Full scope including boiler work: £8,000–£18,000
My plumber's total (two invoices plus additional work) came to £9,695, including fitting a Quooker boiling water tap, against a £10,400 quote. The quote had included a water softener, which I took out of his scope and bought myself for £700, so plumbing overall came to £10,394.
Plumbing is where costs can spiral. Every additional appliance (island tap, water softener, wine cooler) adds a pipe run and connection. Scope this carefully.
8. Screed
If you're installing underfloor heating (which I'd recommend for a ground floor extension), the UFH pipes need to be embedded in screed. Liquid screed is the standard choice for UFH.
- Liquid screed (50m²): £2,000–£3,000
- Traditional sand-and-cement screed (50m²): £800–£1,500
My screed specialist cost £2,622 for liquid screed. The builder had quoted a provisional sum of £1,300, roughly half the actual cost. Provisional sums for screed are almost always optimistic.
Budget for liquid screed at its actual rate: £18–£34 per m². Don't let a builder's provisional sum mislead you.
9. Plastering
Screeding and plastering follow each other in sequence. Once the screed has dried (allow several weeks for liquid screed), the plasterer plasterboards the walls and ceiling and applies skim coats.
- Plastering a kitchen extension: £1,500–£3,500 depending on size and complexity
On my build the extension's plasterboard and plastering materials came through the builder's invoices, so they sit inside his figure. The only separate plastering payment was £250, for bonding and plastering upstairs bedrooms.
10. Tiling and Flooring
Flooring and tiling is where taste-driven cost variance is highest.
- Porcelain floor tiles (supply and labour): £60–£120 per m²
- Wall tiles (supply and labour): £50–£100 per m²
I chose large-format 60x60 Bravestone Gray porcelain tiles. With 81m² of floor space and some wall tiling, the total came to £7,033: £3,933 for the tiles themselves (bought with a 20% trade discount) and £3,100 in labour.
For a 25m² extension with a reasonable tile choice:
- Floor tiling (25m²): £1,500–£3,000
- Kitchen wall splashback tiles: £500–£1,500
Total tiling and flooring: £2,000–£5,000 for a typical kitchen.
11. Kitchen Fit-Out (Units, Worktops, and Fitting)
This category is entirely budget-dependent. The kitchen units themselves are a separate purchase (not included in any builder's quote), and the cost range is enormous.
- Kitchen fitter labour: £1,500–£3,500
- Kitchen units and appliances: £3,000 (flat-pack, self-sourced) to £50,000+ (fully bespoke)
- Stone worktop templating and fabrication: £1,500–£4,000 depending on material and length
The kitchen fit-out is the part of the project most directly under your control. Our kitchen (sourced through an independent kitchen designer) was in the mid-range. The fitter's labour was part of a wider arrangement.
For budgeting purposes, a mid-range kitchen for a 25–30m² extension runs £8,000–£20,000 including units, worktops, and fitting, but not appliances. Allow separately for appliances.
12. Decoration
Once the plastering has dried and cured (at least 4 weeks, ideally longer), the decorator applies a mist coat and two finish coats.
- Decoration (painting) for a kitchen extension: £1,000–£2,500
My decoration came to £2,188, spread across the kitchen, utility room, and some upstairs rooms affected by the build.
13. Garden Reinstatement
Construction destroys the garden behind your house. Temporary fencing, concrete, mortar spills, and equipment storage leave the patio area in a state. Budget for making it good.
- Patio (supply and lay): £60–£120 per m² depending on material
- Garden clearance and reinstatement: £2,000–£8,000 depending on scope
My landscaper quoted £6,350 for the patio, then £7,000 once we chose to relay the old sandstone. I paid in cash as the work went along, so it never reached the build ledger and is not in the totals in this post. The ledger's own garden and drive costs were £536: gravel for the front drive, which was needed partly because the gas trench ran through it, and the excavator's off-hire. The soakaway is counted with drainage under utility moves in section 5.
14. Waste Disposal
During construction, you will fill multiple skips. The builder typically provides some skips within their quote, but extras are often charged separately.
- Builder's skip (6–8 yard): £200–£500 each
- Total waste budget for a full extension: £800–£1,500
The builder supplied most of my skips inside his price. Outside it I paid £590: £300 for same-day removal of the old kitchen floor, a £240 skip for kitchen and garden waste, and £50 towards a shared skip.
What Drives Cost Up
In my experience, four factors blow extension budgets:
1. Steelwork complexity. A simple opening is cheap. Corner posts, cantilevers, and structural changes to upper floors are expensive. Worse, steelwork problems compound: once my builder made unauthorised changes to the spec, it cascaded into months of delays and additional fabrication costs.
2. Glazing choices. There is a £15,000+ gap between standard doors and a premium bifold setup. Decide early and budget accordingly.
3. Utility complications. If your meters need moving AND you discover utility pipes in the dig, costs escalate quickly. My gas pipe was struck twice during groundwork. This wasn't anyone's fault; the utility plans were inaccurate and it cost several hundred pounds and days of delay.
4. Provisional sums. Builders use provisional sums for work they can't precisely price at quote stage: steelwork, screed, glazing, drainage. These sums are almost always optimistic. My screed provisional sum was £1,300; the actual cost was £2,622. Treat every provisional sum as a minimum, not a budget.
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The 2026 Cost per Square Metre
| Category | Typical Range (2026) |
|---|---|
| Builder's structure only | £1,100–£1,500/m² |
| All-in (excluding kitchen units and appliances) | £2,200–£3,000/m² |
| All-in (including mid-to-premium kitchen package) | £2,500–£3,600/m² |
These figures are based on UK national averages. London adds 20–30% to most trade costs.
How Much Contingency Do You Really Need?
Every guide says budget 10–15% contingency. My experience says this is not enough.
My build overran by £76,650 against the builder's original quote, though it's fairer to say the original quote never covered the full scope. When I look at unexpected costs only (the ones nobody saw coming), the overruns included:
- Steel changes on site (rear beams altered to fit, and a welded plate building control suggested): £1,405
- Insulation upgrade required by BCO: £773
- Bifold supplier insolvency: the replacement order cost £3,529 more than the original (the new one also included integrated blinds)
- Screed provisional sum shortfall: £1,322
That's £7,029 in genuinely unforeseeable overruns, about 15% of the builder's quote. Two things do not belong on the list. The emergency gas reroute replaced the scheduled job, which was refunded, so it cost only £48 more. And the cantilevered steel that removed a corner post (£1,535 with the engineer's calculations) was a design change I chose mid-build, not a surprise. Scope changes are the other half of every overrun, and they are the half you control.
Budget 20% contingency on the total all-in figure, not just the builder's quote. On a £80,000 project, that means keeping £16,000 in reserve. It feels like a lot. By the end, you will wish you had it.
The Complete Budget Summary
| Category | Typical (25m²) | My Build (55m²) |
|---|---|---|
| Professional fees (architect, engineer, planning, building control, build-over) | £3,000–£5,000 | £3,785 |
| Builder's structure (excluding steel, counted in the next row) | £28,000–£40,000 | £44,077 |
| Steelwork (supply, fabrication and install) | £3,000–£6,000 | £8,645 |
| Glazing (bifolds, windows, rooflights, lead flashing) | £4,000–£12,000 | £30,019 |
| Utility moves and drainage | £0–£6,000 | £5,128 |
| Electrics | £3,000–£6,000 | £6,624 |
| Plumbing (including the water softener I bought) | £5,000–£12,000 | £10,394 |
| Screed | £1,500–£3,000 | £2,560 |
| Plastering (outside the builder's invoices) | £1,500–£3,500 | £250 |
| Tiling and flooring | £2,000–£5,000 | £7,033 |
| Decoration | £1,000–£2,500 | £2,188 |
| Garden and drive (see note) | £2,000–£8,000 | £536 |
| Waste disposal (outside the builder's skips) | £800–£1,500 | £590 |
| Doorbells (bought direct) | – | £320 |
| Total (construction only, ex-kitchen) | £55,000–£75,000 | £122,150 |
| Kitchen units, appliances, worktops and fittings | £8,000–£25,000 | £59,766 |
| True all-in total | £62,500–£90,000 | £181,916 |
My build column is every paid line of the build ledger, grouped into these categories, and it sums to the two totals (rows are rounded to the pound). Figures are 2021–2024 prices. Adjust upward by roughly 10–15% for building work (ONS output prices for housing repair and maintenance rose about 12% from October 2021 to June 2026), and by more for materials bought on their own, which rose about 18% from October 2021 to July 2026; the build cost index has the latest month and restates an old quote at current prices. Three rows look low because the cost sat somewhere else: the extension's plasterboard and plastering materials came through the builder's invoices, the builder supplied most skips, and the garden is missing. The landscaper quoted £6,350, then £7,000 once we chose to relay the old sandstone, and was paid in cash as the work went along, so the patio is not in the ledger or in these totals. Glazing is net of the failed supplier's refund, and screed is net of a small overestimate refund. The typical totals run narrower than the stacked category maximums because no single build hits the ceiling of every category at once; the wide category ranges show where individual choices (glazing, kitchen, garden) can push a line item up.
What I Would Do Differently
Get a full scope breakdown before signing anything. My builder's quote covered his structural work. Everything else (electrics, plumbing, glazing, kitchen fitting) was either listed as a prime cost sum on top of his total or simply not on the quote. I didn't realise the true scale of what was missing until the invoices started arriving.
Always ask your builder for a line-by-line breakdown, then check each line against the categories above. If anything is missing, ask where it sits in the budget.
Budget based on what you actually want, not what's vaguely implied by a headline figure. The bifold doors that turn a kitchen extension into something special cost £10,000–£25,000 more than a standard French door setup. If you want them, budget for them explicitly. Don't hope the builder's provisional sum will cover it.
Keep 20% contingency, not 10%. Construction throws surprises. Ground conditions, structural discoveries, BCO requirements, supplier failures: any of these can add thousands. A 20% contingency on the all-in figure gives you room to absorb them without crisis.
For a full walkthrough of the sequence, what happens at each stage, and what building control inspects, see our guide on the complete timeline for project managing a kitchen extension.