What Affects the Cost of a Kitchen Extension in London

Why there is no reliable cost per square metre

A kitchen extension is one of the hardest projects to reduce to a rate per square metre, because so little of the spend is the floor area itself. The same-sized rear infill can cost very different amounts depending on the structural opening it needs, the glazing you choose, how far the drains and services have to move, and the kitchen that goes into it. A headline figure that ignores those variables is more likely to mislead than to help.

We keep this stance deliberately, and it is worth stating plainly: we do not publish a build cost per square metre for a kitchen extension. A dependable number comes from a resolved design and a competitive tender, not from a rule of thumb — and the sections below explain what actually moves the figure.

The footprint: rear, side-return infill or wraparound

The shape of the extension is the first cost driver, because each type involves a different amount of new structure and roof.

A single-storey rear extension adds depth across the back of the house. A side-return infill fills the narrow, usually redundant alley beside a period back-addition — a modest footprint that transforms a dark galley kitchen into a full kitchen-diner. A wraparound combines both and is the most generous, and the most involved, of the three.

A wider opening, more roof and more external wall generally mean more cost, but the return is usable, connected living space — which is why the side-return remains one of the most cost-effective moves in a London terrace.

The structural opening to the rear

Turning the back of the house into open-plan space is where a large share of a kitchen extension budget goes, and it is largely invisible in the finished room. Removing the original rear wall — often load-bearing, and sometimes carrying the floors above — needs new beams, padstones and, on a side-return, careful support of the party wall structure.

The wider and more open you want the space, the heavier the structure, and the more structural engineering, building-regulations detailing and temporary support the build involves. This is not an area to economise, and it is one of the main reasons two apparently similar extensions can be priced very differently.

Glazing: rooflights, sliders and roof glass

Glazing is where a kitchen-diner earns its light, and where specification drives cost as much as size. A side-return infill in particular relies on rooflights or a glazed roof to light the middle of a deep-plan room, because the walls are largely enclosed by neighbours.

Common choices that each carry a different cost:

• Rooflights or a run of roof glass to bring light deep into the plan
• Large sliding or bi-fold doors to the garden, where the size and quality of the system matter as much as the opening
• Structural or frameless glazing, which is striking but adds both glazing and structural cost
• Slimmer, higher-performance frames and glass, which lift both comfort and the figure

The kitchen fit-out itself

It is easy to budget the extension and forget that the reason for it — the kitchen — is a substantial cost in its own right, and one that varies enormously. Cabinetry from a standard range and a bespoke joiner's kitchen are different orders of cost, and the worktops, appliances, tap and sink, splashbacks and lighting all compound.

Because the kitchen is fitted by different suppliers on very different budgets, it is one of the largest sources of variation between two otherwise identical projects. Deciding the level of the kitchen early — and pricing it as part of the whole — avoids the common shock of a build cost that excluded the room's entire purpose.

Drainage, services and the moves you cannot see

Extending over the back of a London house almost always disturbs services. Existing soil and rainwater drains, and sometimes a manhole, may sit exactly where the new floor goes, and moving or building over them adds cost and, occasionally, requires agreement with the water authority.

A kitchen also concentrates services: gas or electric supply for cooking, hot and cold feeds, waste, extraction to outside, plentiful power and often underfloor heating in the new slab. Relocating the kitchen to the far end of a new extension lengthens every one of those runs. None of this is visible in the finished room, but all of it is in the price.

Ground, access and logistics

Two practical factors quietly shape a builder's number. The ground determines the foundations: tree roots, made ground, a high water table or a neighbouring basement can all mean deeper or more engineered foundations than a straightforward footing.

Access does the rest. Many London kitchen extensions can only be reached through the house or a narrow side passage, so materials come in and spoil goes out by hand or through the property. Restricted working hours, protection of neighbours and nowhere to store materials on a tight site all affect the price. A sensible contingency — particularly in a period home, where opening up the rear can reveal the unexpected — protects against surprises a survey cannot wholly rule out.

Planning, party wall and the fees you can pin down

Beyond the build, a kitchen extension carries professional and statutory costs — and here, unlike the build cost, some numbers are fixed and knowable.

Many single-storey rear and side-return extensions are permitted development, needing a Lawful Development Certificate (statutory fee £274) to confirm it rather than a full application. Where planning permission is required — common in a conservation area, under an Article 4 direction, or beyond permitted-development limits — the statutory householder application fee is £548 (from 1 April 2026), the same in every London borough because it is set by government. A separate Planning Portal service charge and building control fees also apply.

Because a kitchen extension usually builds up to or astride a party wall, and often excavates near a neighbour's foundations, the Party Wall etc. Act frequently applies — sometimes to neighbours on both sides — which adds surveyor costs and time to budget for from the outset.

Getting to a real number

The dependable route to cost certainty is a design that resolves the structure, glazing, drainage and kitchen, then a competitive tender from a complete tender package — coordinated drawings, schedules and an outline specification — so suitable contractors price the same scope and their quotes are genuinely comparable.

Our own fees are fixed and quoted before you commit, starting at a £395 route review (credited if you proceed) and rising with the scheme; they are set out in full on the packages page. If you would like the fixed numbers for your project, send us the address, the property type and what you are considering, and we will confirm the likely route and a fixed fee.