What Actually Drives Kitchen Extension Costs in London

Last updated: August 2026 · Verified against current government planning rules and London council records.

Why can't anyone give me a straight price for a kitchen extension?

Quick answer: Because two kitchen extensions with the same footprint can land a long way apart on price, any single figure you read online is a guess about your house. The real drivers are the structural opening, the glazing area, the roof type, drainage moves, the finish level and the kitchen itself — not the square metres. The only costs that are fixed and knowable before anyone visits are the statutory ones, such as the £548 householder planning application fee. Everything else deserves a site visit and a fixed, itemised quote.

Why generic cost-per-square-metre figures mislead

Most kitchen extension cost articles work backwards from a per-square-metre rate. The problem is that the rate assumes a specification, and specification is where kitchen extensions swing hardest. Twenty square metres on the back of a Haringey Victorian terrace with a flat roof, two rooflights and French doors is a different project from the same twenty square metres with a full-width sliding door, a glazed roof section and polished concrete floors — same footprint, very different number.

Ground conditions add a layer those articles never mention. Much of North London sits on shrinkable clay, and gardens in Barnet and Enfield often have mature trees near the build line, which can push foundations deeper than any generic figure assumes. Victorian drainage runs where it was laid in 1895, not where your extension wants it. Access matters too: a mid-terrace in Islington or Camden with no side access means every barrow of spoil comes through the house, which changes how the job is priced and programmed.

Finally, most published figures quietly exclude the kitchen itself. Units, worktops and appliances are usually a separate budget from the shell, so a headline cost that does not say whether the kitchen is included is not telling you much. Our kitchen extensions page sets out how we separate shell, services and fit-out so nothing hides between the lines.

The six factors, ranked by how hard they move the number

If you want to understand your own quote before it arrives, these are the levers in rough order of force.

1. Structural opening size. The wider the opening between the old kitchen and the new space, the bigger the steelwork — and a fully open corner or full-width span can need a box frame or goalpost arrangement rather than a single beam. On most jobs this is the largest single swing in the build cost.
2. Glazing area. Sliding and bifold doors are priced by the metre and by specification, and rooflights, roof lanterns and structural glazing all sit at different points. Swapping a standard door set for a full-width slider moves the quote more than most people expect.
3. Roof type. A flat roof with rooflights is the baseline. A pitched roof, parapet detailing or a glazed section adds structure, labour and weatherproofing complexity, and on a 1930s semi the junction with the existing roof needs careful design.
4. Drainage moves. Relocating a soil stack, rerouting an old drain run, or building over or close to a public sewer — which needs an agreement with Thames Water — all add real cost that no per-metre rate captures. This is the factor most likely to surprise people, and the reason a proper site visit beats a phone estimate.
5. Finish level. Underfloor heating, flooring choice, the difference between painted plasterboard and shadow-gap detailing — finish decisions compound across every surface in the room.
6. The kitchen itself. Units and appliances range wider than any other element of the project, which is exactly why they should sit as their own itemised section of the quote rather than be smudged into a single headline figure.

Which costs are fixed and knowable today?

While the build cost needs a survey, the statutory and third-party costs are published figures. These you can put in a spreadsheet this afternoon.

Item	Cost	Notes
Householder planning application	£548	Fee from 1 April 2026; decision due within 8 weeks of validation
Larger-home prior approval	£249	For a 6m rear extension on a terrace or semi (8m detached) under the neighbour consultation route
Lawful Development Certificate	£274 / £548	£274 for proposed works, £548 for existing works
Planning Portal payment charge	£91.02	Added when paying through the Planning Portal
Party wall surveyors	£1,000–£2,500 per affected neighbour	Only where surveyors are appointed; written consent costs nothing beyond the notices

Whether you need planning permission at all depends on size: a single-storey rear extension up to 3m on a terraced or semi-detached house (4m on a detached) is usually permitted development, extendable to 6m or 8m through prior approval. Our planning fees guide walks through which route fits which project. Most North London kitchen extensions also touch the Party Wall etc. Act 1996 — we serve notices for clients as standard and run the party wall clock in parallel with the 8-week planning period, so neither adds time to the other. The party wall guide covers notice periods and what happens if a neighbour dissents.

How a fixed, itemised quote protects you

A vague estimate is a number shaped to win the job, with the detail arriving later as extras. A fixed, itemised quote works the other way: every element is priced line by line — foundations, steel, glazing, roof, drainage, services, finishes — with the kitchen fit-out shown separately and anything excluded stated in writing. If ground conditions genuinely cannot be known until excavation, that is flagged as a named provisional item rather than buried in a round number.

That structure matters when you compare quotes. Two builders quoting the same headline figure may be pricing very different jobs, and the only way to see it is on paper. Every project we take on gets a fixed, itemised quote after a free site visit — that is the policy, not a promotion, and it is why we do not publish per-square-metre rates.

How do I get a realistic number for my house?

Start with our extension cost guide for orientation on what shapes budgets across North London. Then use the Extension Builder — sixty seconds of inputs gives you a guide price for your exact size and choices, which is a far better starting point than a national average. When you are ready for a real figure, book a free site visit and we will measure, check the drainage and boundaries, and return a fixed, itemised quote you can hold us to.

Frequently asked questions

Do I need planning permission for a kitchen extension?
Often not. A single-storey rear extension up to 3m (terraced or semi-detached) or 4m (detached) is permitted development, extendable to 6m or 8m via prior approval. Flats and maisonettes have no householder PD rights, and conservation areas or Article 4 directions — common in Camden and Islington — can remove specific rights, so check before assuming.

Is the kitchen included in the quote?
In ours, it is itemised separately so you can see exactly what the shell, services and fit-out each cost, and adjust one without guessing at the others. Always ask any builder the same question — it is the most common gap between two quotes that look similar.

What will the party wall side cost?
If your neighbours give written consent, nothing beyond the notices themselves, which we serve for clients as standard. Where surveyors are appointed, budget £1,000 to £2,500 per affected neighbour; the building owner pays.

Why won't you publish a price list?
Because the same footprint prices differently on different houses — clay and tree roots in one garden, a shared drain in another, no side access in a third. A published rate would be wrong for most readers, so we price each job individually after a free site visit.

How accurate is the Extension Builder guide price?
It is a guide built from your dimensions and choices — genuinely useful for orientation and for comparing options, but it is not a contract figure. The fixed, itemised quote that follows the site visit is the number we stand behind.

Does a bigger extension always cost more?
Not proportionally. An extra half-metre of footprint moves the number far less than a wider structural opening, more glazing or a drainage relocation. Spending on the right factors matters more than the raw size.