What a rear extension really costs in 2026
Most people start with a number in their head and a rough size in mind. The trouble is that the two are connected in a way that surprises people: the first metre of an extension is by far the most expensive one you buy, and every metre after it is cheaper than the last.
Understanding where the money goes makes it much easier to read a quote, and much harder for a cheap one to catch you out.
Why the first metre costs the most
Before a single square metre of floor exists, an extension needs foundations dug and filled, drains diverted or built over, a structural opening formed in the back of the house, a roof, a damp-proof course, and a Building Control inspection at every stage. Those costs land whether you build out two metres or five.
Stretch the same extension a metre further into the garden and you add some more foundation, a bit more brickwork, a bit more roof. The expensive one-off work is already paid for.
This is why a small extension can feel poor value per square metre and a larger one can feel like a bargain, even though the larger one costs more in total. If your budget is close to the line, going slightly bigger is often better value than going slightly smaller.
If you are choosing between 3m and 4m and the difference in price is small, it is almost always the extra metre that is cheap, not the first three that were expensive.
What you are actually buying
A single-storey rear extension is usually quoted somewhere between £55k and £110k, and a side return infill between £45k and £85k. Those are guide ranges for typical London work including VAT, and the spread inside them is wide for good reasons.
Roughly, the money goes on:
Groundwork and foundations, which vary more than anything else on the job. Structure, meaning the steel beams and the opening into the existing house. The shell, which is brickwork, roof and weatherproofing. Then first and second fix, which is electrics, plumbing, plastering and decoration.
None of those is where people usually overspend. The overspend is almost always in glazing, kitchens and floor finishes, which are the parts you choose rather than the parts you need.
The five things that move the number most
Ground conditions. Foundations are priced on assumptions until somebody digs. Clay soil, made ground or a high water table all mean going deeper, and deeper foundations cost more in both concrete and time.
Trees. A tree near the extension, even in a neighbour's garden, usually means deeper foundations to cope with how the soil moves as roots take up water. It is one of the most common reasons a groundwork price moves after the survey.
Drains. A shared drain or public sewer running under the footprint usually means a build-over agreement with the water company, and sometimes a diverted run or a new manhole. Both are ordinary work, but both cost money and take time to agree.
Glazing. Bifold doors, sliders and structural glazing cover an enormous price range for openings that look similar on a drawing. This is the single easiest place for two quotes to differ by thousands while appearing to describe the same extension.
The kitchen. Most rear extensions end up containing one. Units, worktops and appliances are usually bought separately by the owner, so whether they appear in a quote at all varies from builder to builder.
What usually sits outside the quote
On our own extensions page we list this in full, but the items that most often surprise people are:
Scaffolding, which is surveyed and priced by a scaffold contractor rather than guessed at. Architect drawings and structural calculations, if you do not already have them. Party wall surveyor fees, if your neighbours appoint one, which our guide to party wall notices explains. Kitchens, glazing and landscaping outside the new walls.
None of these are hidden costs when they are written down. They become hidden costs when a quote stays quiet about them and the conversation happens later.
How to compare two quotes fairly
The most useful thing you can do is make both builders price the same extension. That means deciding the things you would rather leave until later:
Pick the glazing, or at least set an allowance and tell both builders the same number. Decide the floor finish. Say whether the kitchen is in or out. Confirm who is arranging the drawings and the structural calculations.
Then read what each quote says about the parts nobody can see yet. A quote that names its foundation assumption and says what happens if the ground turns out to be different is more honest than one that quotes a single confident number for groundwork it has not dug.
"What would have to be true on site for this price to change, and by roughly how much?" A builder who has thought about your house can answer that in a couple of sentences.
What to do before you ask for prices
Get someone to look at the back of the house, the garden level, the drain covers and the trees before you commit to drawings. Most of the things that move an extension price are visible in half an hour on site, and all of them are cheaper to find out about now than in week six.
A survey will tell you what your house can take, what the ground is likely to do, and which of the choices above are actually worth spending money on. If you would like one, book a survey and we will come and measure up.