Two neighbours got kitchen extensions the same year. One spends £55,000, the other £130,000, and both have a 20 square metre rear extension. The difference is not the shell, which costs roughly the same for both. It is what goes inside it, and how many professionals touched it. A kitchen extension is really three separate budgets stacked together, and understanding where each pound goes is the only way to compare quotes honestly.
Those three budgets are the build, the fit out, and the fees. Here is what each one actually costs in 2026.
Budget One: The Build (The Shell)
This is the extension itself. Foundations, walls, roof, glazed doors, flooring, plastering, electrics and heating. Everything that makes it a room.
In London, the build runs roughly £2,500 to £4,000 per square metre for a standard to good specification, before the kitchen goes in. A 20 square metre rear extension therefore lands around £50,000 to £80,000 on the shell alone.
Side returns cost more per square metre, often £3,000 to £4,500, because you build three walls and a roof for a small floor area on a technically awkward Victorian terrace. Rear extensions are more economical per metre, because the cost spreads over a larger footprint.
Nationally, outside London and the South East, the same build sits 20 to 40 per cent lower.
Budget Two: The Fit Out (The Kitchen Itself)
This is the part a general extension cost guide ignores, and it is the single biggest reason two kitchen extensions differ in price.
The kitchen fit out, meaning units, worktops, island, sink, tap, appliances and tiling, typically accounts for 25 to 30 per cent of the total project. It is also the part you control most, because the range is enormous.
| Kitchen tier | Units, worktops and appliances |
| Budget (flat pack, laminate worktops, standard integrated appliances) | £8,000 to £15,000 |
| Mid range (rigid cabinets, quartz worktops, integrated appliances) | £15,000 to £30,000 |
| Premium (bespoke cabinetry, natural stone, high end appliances) | £30,000 to £60,000+ |
Worktops alone swing wildly. Laminate runs £40 to £80 per linear metre. Solid wood or composite, £2,500 to £5,000 for a kitchen. Premium quartz or granite can top £600 per linear metre.
This is where the neighbour who spent £130,000 pulled away from the one who spent £55,000. Same shell, very different kitchen.
Budget Three: The Fees (The Invisible Third)
The costs that never appear in a per square metre rate, and the ones people forget to budget.
Design and professional fees run 7 to 15 percent of build cost, covering architectural drawings and structural coordination. A structural engineer sizes the beams and foundations, from £500 upward, and building control will not sign off without the calculations.
Statutory fees are fixed. The householder planning application fee in England is £548 from April 2026, or £249 for prior approval on a larger extension. Many kitchen extensions are permitted development and need neither, only a Lawful Development Certificate at £274 if you want written proof.
Party wall surveyor fees apply where a neighbour dissents, typically £1,000 to £3,000 per neighbour on a terrace.
And a contingency of 10 to 15 per cent, because older houses reveal shallow foundations and half brick walls once opened up.
What Does a Kitchen Extension Cost in London in 2026?
Put the three budgets together and a typical London kitchen extension costs £75,000 to £150,000 all in, for a 20 to 30 square metre rear extension including the build, the kitchen, glazing, fees and VAT.
Broken down, that is roughly £50,000 to £80,000 on the shell, £15,000 to £35,000 on the kitchen fit out, and £8,000 to £15,000 in professional fees, before VAT at 20 per cent.
A side return comes in lower on the shell, a wraparound higher. The kitchen and fees scale with your choices rather than the footprint.
A Worked Example
A mid range 20 square metre rear kitchen extension in London, 2026.
Shell at £3,000 per square metre: £60,000. Mid range kitchen fit out: £20,000. Professional fees, structural and building control: £11,000. That is £91,000 before VAT, and around £100,000 to £109,000 once VAT applies to the elements it falls on.
Swap the mid range kitchen for budget and you save £10,000. Swap it for premium and you add £20,000 or more. The shell barely moves. The fit out is the lever.
Why Quotes Vary So Much
When two builders quote wildly different numbers for what sounds like the same job, it is almost always one of three things.
They have assumed different kitchen tiers, often without saying so. One priced a £10,000 flat pack kitchen, the other a £28,000 bespoke one.
They have included or excluded the fees differently. A shell only quote looks cheaper until you add the £11,000 of professional costs the other builder bundled in.
Or they have made different assumptions about the ground. A quote with no contingency is not cheaper, it is less complete, and the gap appears mid build.
Comparing the headline number is meaningless. Comparing the three budgets separately is the only way to see which quote is actually lower. A proper breakdown of the kitchen extension cost by element is what lets you do that.
Where the Money Is Worth Spending
Not every pound returns the same value, and experience points to where it counts.
Spend on the glazing and the daylight strategy, because a bright room is what makes an extension feel worth the money, and a dark one never recovers, however good the kitchen. Spend on the structural opening if it buys you a genuinely open plan space. And spend on worktops and the island, which are what people touch and see every day.
Save on appliance brands where the mid range performs as well as the premium, and on finishes that date quickly.
Extension Architecture handled a project on Malden Road in Kingston where the value came from how the ground floor connected to the garden, not from the square meterage or the appliance list. That is the pattern worth copying. The extension that returns best is the one where the money went into light, flow and the structural moves that open the space, rather than into the parts a buyer never notices.
Budget the three separately, decide which tier of kitchen you actually want before you compare quotes, and the real cost of your extension stops being a mystery.
