How Much Does a Kitchen Extension Cost?
A building and a fitted room, priced by different trades and added together far too late.
A kitchen extension has two budgets. The building work runs about £2,200 to £3,000 per square metre, so a 20 square metre extension is roughly £44,000 to £60,000, and a well made mid range kitchen for a room that size is commonly another £15,000 to £30,000 installed. A builder's quotation almost never includes the second figure. Between the two sit the services: a new waste run at the right fall, an extract route for an island hob, new electrical circuits, underfloor heating that has to be decided before the screed goes down, and the load the extra heated space puts on the existing boiler.
Two numbers, not one. The building work runs about £2,200 to £3,000 per square metre, so a 20 square metre kitchen extension is roughly £44,000 to £60,000, and the kitchen that goes in it is commonly another £15,000 to £30,000 installed. A builder’s quotation almost never includes the second figure, and that is where kitchen extension budgets come apart.
Every other cost question on this site is about a building. This one is about a building and a fitted room, priced by different trades, bought at different times, and routinely added together far too late.
| What it covers | Which budget it sits in |
|---|---|
| Shell, structure, roof, glazing | The builder, at £2,200 to £3,000 per square metre |
| Units, worktop, appliances, fitting | The kitchen, at £15,000 to £30,000 installed |
| Drainage, extract, electrics, heating | Neither, so it lands on whichever is still open |
| VAT, flooring, decoration, fees | Outside both |
The second budget, and why it is so wide
A well made mid range kitchen for a room of this size sits in that range installed. It is entirely possible to spend triple, and entirely possible to spend half.
Very little of the spread is the cabinets. It is the worktop material, the appliance specification and the amount of joinery that is not a standard carcass. A composite or quartz worktop with a run of drainer grooves and a shaped island end is a different order of cost to laminate, and it is measured and templated after the units are set, which is why it lands late in the programme and late in the invoicing.
Appliances are the other open end. Integrated everything, a large range, a boiling tap and a wine cooler will each pull the figure up on their own, and they also pull up the electrical work behind them. Decide the rough kitchen figure early, before the building is priced, because the two budgets compete for the same total.
The services that sit between the two budgets
This is the part that belongs to neither quotation and gets billed to whichever one is still open.
A kitchen extension moves water, waste, gas, electricity and air. None of that is unusual, all of it costs money, and it is the classic source of a variation halfway through.
- Drainage. Moving a sink to the far side of a new room means a new waste run at the right fall, and in an older house the existing gully or stack may not be where you would like it. Where the extension crosses a shared drain, a build over agreement and sometimes a camera survey come with it.
- Extract. An island hob needs a duct run and a route out through a roof or a wall, decided at drawing stage. A recirculating hood is cheaper and does not remove moisture, which matters in a room this size.
- Electrics. A kitchen of this scale is several new circuits, and on an older installation the existing consumer unit may not have the way for them. This is notifiable work and has to be certified.
- Heating. Underfloor heating is the usual answer in an open plan kitchen, because the walls are units and glass. It goes in before the screed, so it is decided early or not at all, and it needs its own zone or the room will fight the rest of the house.
- The boiler. Adding twenty or thirty square metres of heated space, much of it glazed, increases the load on a system that was sized for a smaller house.
The building half prices like any single storey extension
There is nothing about a kitchen that changes the way the shell is costed. The variables are the same ones, in the same order: foundation depth on shrinkable clay, access, the width of the structural opening, glazing specification and roof type. Rather than repeat them, they are set out in what a single storey extension costs.
Two of them bite harder here than on a plain rear extension, because of what the room is for. Kitchen extensions tend toward wide openings and a lot of glass, since the point is usually one open room across the back rather than a separate space. A full width structural opening and a run of sliding doors are both at the expensive end of their own ranges, and they are chosen for the same reason.
The daylight strategy is also a cost decision rather than an aesthetic one. Extend deeply across the back and the old middle of the house loses its window, so rooflights over the line of the removed wall stop being a nice extra and start being the thing that makes the plan work. The options for roof glazing vary by an order of magnitude in price.
What neither quotation covers
VAT, flooring, decoration, professional fees for the drawings and the structural calculations, planning and Building Control fees, party wall surveyor fees where a neighbour dissents, and service diversions. The complete list sits on the costs page.
Flooring deserves a mention of its own here, because in a kitchen it is a large continuous area, usually tiled or engineered timber, and it is often specified late from what is left. In a room where the floor runs from the old house through to the doors without a threshold, it is one of the more visible things in the finished job.
Sequencing, which is a cost as well as a nuisance
The extension is built outside the existing wall, so the old kitchen keeps working for most of the programme. The period without one starts when the rear wall comes out and ends when the new kitchen is connected, typically three to five weeks. That is a real household cost in eating out and workarounds, and it is worth asking for the figure before you commit rather than discovering it.
Order the kitchen too late and it becomes the item everybody waits for. Order it too early and it sits in a garage while the worktop template goes out of date. Units are set after plastering and after the floor screed has dried, and the worktop is templated off the fitted units, so those two dates drive everything.
If the total is fixed
Most people arrive with a number for the whole thing rather than for the building. When that number is tight, the honest trade is to build the shell properly and buy a simpler kitchen, because the kitchen can be replaced in fifteen years for a fraction of the cost of correcting a foundation, a roof or an opening that was underbuilt to release budget.
Wide openings, roof glazing and underfloor heating are the same argument. They are cheap to include and expensive to add later. Worktops and appliances are the opposite.
Turning it into a figure
Floor area, ground conditions, access, the width of the opening, the glazing, and a realistic figure for the kitchen itself. With those, both halves can be priced properly and added together once rather than twice. How a kitchen extension is planned and built covers the rest.
These are indicative for the Colchester area at the review date below and move with materials and labour. Treat them as a way of testing a quotation, not as a quotation.
Last reviewed August 2026.
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