Almost every kitchen extension quote a homeowner receives is shaped by three choices made before anyone draws a plan. Depth, roof and consent route. Everything else is finish.
Understanding how these three interact is the difference between a realistic budget and a number that doubles halfway through. Here’s how each option compares.
Decision one: how far back you go
Depth drives cost more than any other variable, because it sets both the footprint and the consent route. It also decides how much natural light reaches the middle of your ground floor.
A side return infill uses the narrow strip beside the original rear wing. It typically adds 8 to 14 square metres, needs modest structural work, and almost always sits inside permitted development.
A full width rear extension removes the whole back wall. It adds more space per pound but needs a much larger steel frame, which is where the structural budget climbs.
A wraparound combines both. It’s the most expensive per project but the cheapest per square metre, because you’re paying for one set of foundations, one roof and one set of drainage diversions.
| Option | Typical area added | Structural demand | Usual consent route |
| Side return infill | 8 to 14 sqm | Low to moderate | Permitted development |
| Full width rear | 15 to 25 sqm | High, long span steels | PD or prior approval |
| Wraparound | 25 to 40 sqm | High, complex junctions | Prior approval or full planning |
Decision two: the roof
This is the choice homeowners underestimate most. The roof determines light, comfort, running cost and around a fifth of the build budget.
A solid flat roof with two or three rooflights is the cheapest and the easiest to insulate to current standards. It gives you controlled light without the heat gain.
A full glazed roof or a large lantern transforms the space visually and is the option most people want after seeing photographs. It’s also the option that pushes up cost, needs more structural support, and creates a room that runs hot in July and cold in January unless the specification is right.
A partial glazed approach, usually a lantern over the dining zone and solid construction over the working kitchen, tends to be the sensible middle. You get the drama where you sit and thermal control where you cook.
| Roof type | Relative cost | Light quality | Thermal performance |
| Solid with rooflights | Lowest | Good, controlled | Easiest to meet Part L |
| Full glazed or lantern | Highest | Excellent | Needs careful specification |
| Partial glazed | Middle | Very good | Manageable |
Decision three: the consent route
Three routes exist, and they carry very different timelines and fees.
Standard permitted development covers a single storey rear extension up to 4 metres for a detached house and 3 metres for a semi detached or terraced house, measured from the original rear wall. No application is needed, though a Certificate of Lawful Development is worth obtaining as proof.
The prior approval route raises those depths to 8 metres detached and 6 metres semi detached or terraced. It requires a 42 day neighbour consultation, and the fee is modest compared with a full application. If a neighbour objects and the council agrees with the objection, it can be refused.
A full householder application is required beyond those depths, on any wraparound that breaches side limits, and anywhere permitted development has been removed. The fee is £548 from April 2026, with an eight week decision target from validation.
Location decides which of these is even available. Conservation areas and Article 4 Directions remove the enhanced depths entirely, which is why work with architects clapham usually starts by establishing the route before the layout is discussed. Lambeth has extensive conservation coverage, and a scheme designed for prior approval is worthless if prior approval doesn’t apply to the address.
How the three decisions compound
The interaction is what surprises people. A 6 metre full width rear extension with a full glazed roof on a terraced house in a conservation area involves the largest steels, the most expensive roof, and a full planning application. Three expensive choices stacked.
The same house, extended 3 metres as a side return with a lantern over the dining table, sits inside permitted development with modest structural work. The finished kitchen may not be much less usable.
On a project off Wandsworth Bridge Road in SW6, Extension Architecture resolved the layout before fixing the structure, which allowed a narrower span and a smaller steel frame while keeping the open plan result the client wanted.
Where the money actually lands
A kitchen extension in London and Surrey typically runs £80,000 to £115,000 including the kitchen itself. Single storey construction usually falls between £2,100 and £3,000 per square metre depending on specification.
Roughly speaking, shell and structure take half, the kitchen and appliances a third, and glazing, finishes and professional fees the rest. Well resolved extensions tend to return 15 to 25 per cent in added value, though that varies with local ceiling prices.
Expect six to eight months from first conversation to completion. Design and consents take a surprising share of that, particularly on the prior approval route where the consultation period alone runs six weeks.
Extension Architecture’s approach is to settle depth and consent route first, then design the layout inside those constraints rather than around them.
The order matters more than the ambition. Establish which consent route your address actually allows, then choose depth within it, then spend the remaining budget on the roof rather than the reverse. Homeowners who design first and check later are the ones who end up paying for two sets of drawings. See more.