How much does an extension cost?
The honest answer is that it depends on more than the size. Two single storey extensions with the same floor area can differ a great deal in price because of what is underneath them, what sits on top of them and what goes inside them. This guide walks through the factors that make the biggest difference, so that when you compare quotes you can see why they differ and what each one includes.
It does not give figures. Any number given without seeing the house, the ground and the drawings would be a guess, and guesses cause problems later. A site visit and a free written estimate are the way to get a real price.
Size, shape and the cost per m2
Floor area is the starting point, and you will often see extension cost quoted per m2. That figure is useful for comparing like with like, but it hides a lot. Some costs are fixed whatever the size: setting up, connecting to the house, drainage connections, and the steel or opening where the extension joins. A small extension carries those costs over fewer square metres, so its cost per m2 is usually higher than a larger one.
Shape matters too. A simple rectangle uses less material and labour than an L-shape, a wrap-around or a design with recesses, because every extra corner adds brickwork, foundations and roof detail.
Single storey, two storey and the main types
A single storey rear extension is the most common project and the simplest structurally. A side return extension fills the narrow space at the side of many terraced and semi-detached houses; it is often small in area but involves party wall matters, a glazed or pitched roof against a boundary and a lot of knocking through. A wrap-around extension combines both, adding corners, a more complex roof and more steelwork.
Two storey extension cost is not simply double. The foundations and roof are shared between both floors, so the extra floor can add useful space for less per square metre, but it brings upstairs layout changes, stairs and bedroom finishes, and usually needs a planning application. Our double-storey extension guide explains what is different.
Ground and foundations
What is under the garden can change the budget more than anything you see. Clay soil, nearby trees, made-up ground and drains under the footprint can all mean deeper or more specialised foundations. Building over or close to a public sewer needs approval from the water company and can mean diverting the pipe. Many of these questions are answered at the start through a site survey, trial holes and the Building Regulations design.
Walls, roof and openings
The walls are usually brick or block cavity construction with insulation, built to match or complement the house. Facing bricks vary in price, and matching an older brick can take some searching.
The roof is a big variable. A flat roof is generally the simplest; a pitched roof matching the house needs more structure and tiling; a lantern or a run of rooflights adds glazing and detailing. Taking out a large section of the back wall to open into the extension needs steel beams designed by an engineer, and wide spans need bigger steels. Large sliding or bifold doors are one of the most visible costs in a modern extension.
Kitchen extension cost and the fit-out
Many extensions are built to make a bigger kitchen, and the kitchen itself is often the largest single item in the fit-out. Kitchen extension cost depends heavily on the units, worktops and appliances, and on moving plumbing, drainage and gas. Underfloor heating, electrics, lighting, plastering, flooring and decoration all add up. When comparing quotes, check which of these are included and which are left to you, because a quote for a shell and a quote to finish look very different.
Fees and approvals
Most extensions need drawings, a structural engineer's calculations and Building Regulations approval. Some need a planning application; others can be built under permitted development, though a lawful development certificate is worth having. If you share a wall or are digging near a neighbour's foundations, the Party Wall Act may apply. These costs sit alongside the build and are easy to forget when setting a budget. Our extension planning guide explains the approval routes.
Access, timing and getting accurate quotes
How materials and machinery get to the back of the house matters. Narrow side access, no driveway or a terraced street can slow a job down. The more complete your drawings and specification are when you ask for quotes, the more accurate and comparable those quotes will be.
If you are planning an extension, we offer free estimates after a site visit and work from your architect's drawings. You can also use our cost calculator to think through the main choices before we visit.
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Questions people ask
How much does an extension cost?
It depends on the size, shape, foundations, roof, glazing, steelwork, fit-out and fees. We do not give figures without seeing the house; a site visit and a free written estimate give you a real price.
Is a two storey extension cost double a single storey?
No. The foundations and roof serve both floors, so the extra floor adds space for less per square metre, but it brings stairs, upstairs finishes and usually a planning application.
Why does extension cost per m2 vary so much?
Fixed costs such as connecting to the house, drainage and steelwork are spread over the floor area, so small extensions cost more per m2. Ground conditions and the specification also change it.
Does the quote include the kitchen?
It varies between builders. Check whether a quote covers only the structure or also the kitchen, plumbing, electrics, plastering and flooring before you compare prices.