Planning a kitchen extension: sequencing structure, services and fit-out

A kitchen extension is really two projects in one: a building project that creates the space, and a kitchen project that fills it. Most of the problems people run into, such as sockets hidden behind tall units, worktops that do not fit or a floor that cracks, come from doing things in the wrong order. This guide walks through the sequence from first sketch to final sign-off, so you know what has to be decided when, and why.

Start with the kitchen layout, not the building

It is tempting to design the extension first and fit a kitchen into it afterwards. It works much better the other way round. The positions of the sink, hob, extractor, fridge and any island decide where drainage, gas, water, extraction ducting and electrical circuits have to run, and some positions are far easier to serve than others. A sink on an island, for example, needs a waste pipe run under the floor with enough fall, which is simple to allow for before the slab is cast and expensive to add afterwards.

So settle the layout, with your kitchen designer if you are using one, before the building drawings are finalised. At the same time, decide on the things that affect the structure and the floor: underfloor heating, the floor finish and its thickness, roof lights or a glazed roof, and where you want the widest openings.

Approvals and lead times

Next come the approvals: planning permission or a Lawful Development Certificate, Building Regulations approval, structural calculations for any steel beams, and party wall notices if you share a wall with a neighbour. Our guides on planning permission for extensions and the Party Wall Act cover those in detail.

This is also the moment to order the kitchen. Kitchen suppliers often have lead times of several weeks, and bespoke joinery or stone worktops can take longer. A good builder programmes the building work backwards from the kitchen delivery date, so the room is ready when the units arrive rather than the other way round.

Local rules that shape the space

Where your kitchen extension needs planning permission, local guidance can decide its size and shape. In Fulham, Hammersmith & Fulham generally resists building above existing back additions, which pushes most schemes to extend at ground floor. Richmond's guidance, which applies in East Sheen, treats single-storey rear extensions as usually acceptable up to about 3.5 metres on a semi-detached house. In New Malden, Kingston's planning policy seeks to keep large gardens and generous plots, so extensions that preserve garden space are better received. Each area page sets out the local position.

Groundworks and structure

On site, the first stages are the heaviest. Any existing structure is stripped out, foundations are dug and poured, and new drainage is laid in the ground, including the runs your kitchen layout needs. The walls go up, steel beams are installed where walls are being removed or openings widened, and the roof is built. Building control inspects the foundations, drainage and structure at key stages.

This is where the back of the house is opened up, often into a side return. Our guide to side-return extensions covers the particular challenges of period terraces, from light to drainage.

Weathertight, then first fix

Once the roof is on and the windows, doors and roof lights are fitted, the extension is weathertight and work moves inside. First fix is when all the hidden services are installed before the walls are closed up: water supply and waste pipes, the gas supply if you have a gas hob, cables for sockets, lighting and appliances, extractor ducting to the outside, and any underfloor heating pipes or mats.

First fix is the point of no return for the kitchen layout. Every socket, switch, pipe and duct is positioned to the kitchen drawings, so a change of mind after this stage means cutting into finished work. It is worth walking through the room with the drawings before the walls are plastered.

Insulation, screed and drying time

The floor is usually insulated and finished with a screed, a smooth layer laid over the insulation and any underfloor heating. Screed needs time to dry before most floor finishes can go down, and that drying time is one of the most common reasons kitchen extensions overrun when it is not planned for. Laying flooring on a damp screed can lead to failed adhesive, lifting boards or cracked tiles. A realistic programme allows for it, and there are faster-drying screeds where time is tight.

Walls and ceilings are then plastered, and plaster also needs to dry before decoration.

Fitting the kitchen

With the room plastered and the floor ready, the kitchen is installed: base units first, then wall units and tall units. Stone and composite worktops are normally templated once the base units are fitted, because they are cut to the exact shape of the installed cabinets, and there is then a wait while they are made. Laminate and timber worktops can usually be fitted straight away.

Whether flooring goes down before or after the units depends on the material and on your preference; running the floor under the units makes future changes easier, while fitting it around them uses less material. Second fix follows: sockets and switches, lighting, the sink and taps, appliances and the connection of the hob by a registered engineer.

Finishing, snagging and certificates

The last stage is decoration, then a snagging walk-round to list and fix any small defects. Before you close the project, make sure you have the paperwork: the building control completion certificate, an electrical installation certificate for new circuits, a gas safety certificate if gas work was done, and any guarantees for the waterproofing, glazing or appliances. You will need these when you sell. Keep them together with the approved drawings and structural calculations, because a future buyer's surveyor may ask to see them all.

Living without a kitchen

Expect to be without a working kitchen for a good part of the project. Most families set up a temporary kitchen in another room with a microwave, a kettle, a fridge and access to a sink, and plan meals around it. A clear programme agreed at the start tells you how long each stage will take, which makes the disruption much easier to manage. Our kitchens and house extensions pages explain how we run a combined project, and we work around your supplier's drawings and delivery dates. Areas like Earlsfield and Putney are full of period houses where a kitchen extension is the most popular project of all.