What a London kitchen extension really costs

The kitchen extension is the defining London home improvement: a dark rear room and a narrow side alley turned into one bright space where the household actually lives. It is also the project where budgets most often come apart, because the headline figure people carry in their heads covers roughly half of what the work involves.

Here is an honest breakdown of what makes up the cost, expressed as broad indicative ranges for 2026, and how to build a budget that survives contact with the site.

The three common forms

Side return

Filling the narrow alley alongside the original rear closet wing. Typically adds eight to fifteen square metres, transforms the light, and requires structural work to the flank wall and often a party wall agreement with the neighbour.

Rear extension

Building out into the garden across the full width. Adds more floor area for the money than a side return and usually involves a single large opening in the rear wall, supported on a steel beam.

Wraparound

Side return and rear extension combined. The most floor area, the most complex roof geometry and drainage, and the greatest structural intervention — priced accordingly rather than as the sum of the two parts.

What actually drives the cost

Structure and groundworks. Foundations to a depth dictated by soil, trees and adjacent drainage; underpinning where the neighbour's foundations are shallower; steel frames where you want an open rear elevation without visible columns. London clay and mature street trees regularly push foundations deeper than expected.

Glazing. The single most variable line. Standard aluminium bifolds and a couple of rooflights sit at one end; slim-framed structural sliding units, frameless corners and a large flat rooflight sit far above it. Glazing is also a long-lead item that can hold the whole programme.

Services. Moving the boiler, upgrading the consumer unit, new drainage runs and manholes, extract ventilation, underfloor heating manifolds and the electrical infrastructure for lighting scenes. Rarely visible in the finished room and never cheap.

Finishes. Floor build-up and screed, resin or stone or engineered timber flooring, plaster, decoration, joinery and the level of detail at junctions. The difference between a good finish and an exceptional one lives here.

The kitchen itself. Cabinetry, worktops, appliances and installation form a separate budget from the building work, and clients almost always underestimate it.

Broad indicative ranges, 2026

For a good-quality London build, treat these as orientation only:

• Shell and core construction: roughly £3,000 to £4,500 per square metre for a straightforward build, rising well beyond that for complex structure, deep foundations or exceptional glazing.
• Side return, 10–15 m², building work: approximately £90,000 to £160,000.
• Full-width rear extension, 20–30 m², building work: approximately £130,000 to £250,000.
• Wraparound, 30–40 m², building work: approximately £180,000 to £320,000.
• Fitted kitchen: approximately £20,000 to £60,000 for a quality made kitchen; bespoke cabinetry and stone can exceed this comfortably.
• Reinstating and decorating adjoining rooms: frequently £10,000 to £30,000, and frequently forgotten.

These figures assume a competent main contractor working to a complete drawing package. Very low quotations usually indicate an incomplete scope rather than a bargain.

The costs that sit outside the builder's quotation

1. Professional fees — architect or designer, structural engineer, building control, and a party wall surveyor for each adjoining owner who dissents. Together these commonly represent a meaningful slice of the construction budget.
2. Planning and certificates — application fees, lawful development certificate, and any surveys the council requests.
3. Party wall awards — you generally pay the adjoining owner's surveyor as well as your own.
4. Access and logistics — scaffold, skips, permits, parking bay suspensions, hoarding, protection.
5. VAT. Most domestic renovation and extension work in the UK is charged at the standard rate. Reduced rates exist in specific circumstances — properties empty for a long period, or works that change the number of dwellings — and they come with conditions. Check your position with HMRC guidance or your accountant, and confirm whether a quotation is inclusive or exclusive of VAT before you compare it with another.
6. Kitchen appliances, lighting fittings, sanitaryware and furniture — usually a client supply, usually underestimated.
7. Living costs — renting elsewhere, storage, eating out during a kitchen-less summer.
8. Contingency. Ten per cent is a minimum on a new-build extension. Fifteen to twenty per cent is prudent where you are opening up an old house and cannot see what you are buying.

How to build a budget that holds

• Set the total figure you are willing to spend first, then design to it. Designing first and pricing later is how projects stall after planning.
• Ask for an early cost plan from someone who prices for a living, not a per-square-metre guess.
• Tender a complete package. Two or three contractors pricing the same drawings and specification gives you comparable numbers; a sketch gives you three different scopes.
• Read exclusions before headline totals. That is where the difference between quotations actually lives.
• Fix the specification before work starts. Changes made on site cost more than the same decision made on paper, and they cost programme as well as money.
• Keep the contingency separate and untouched. It is not a shopping allowance.
• Agree a payment schedule tied to completed stages, and never pay far ahead of the work.

Where the value is

The parts of a kitchen extension you notice every day are light, proportion, storage and the quality of junctions — not the size. A well-planned fifteen square metres will outperform a poorly planned thirty. We work through that discipline as luxury space optimisation, design and detail the room through interior design and styling, and carry the openings, beams and foundations under structural modifications. Programme is held from the first drawing through project management and coordination.

For a realistic view of timescales, read how long a London renovation takes. To see finished rooms rather than figures, browse our projects — then tell us about your property and we will give you an honest budget band before anyone draws anything.

Indicative guidance for 2026 only, not professional or tax advice. Construction costs vary by property, borough, specification and market conditions, and tax treatment depends on your circumstances. Obtain a specific cost plan and confirm VAT treatment before committing.