DATA-BACKED GUIDE · UPDATED JULY 2026
Single storey extension cost UK: the per square metre reality
A single storey rear extension is the classic way to open up a kitchen or add a bright family room. Builders often quote a rough per square metre rate, but that number hides as much as it tells you. It usually covers the shell and little else, so two homeowners can be given the same rate and end up with bills thousands apart once the kitchen, glazing and groundworks are added. The real prices on this page come from what people have actually paid, so you can sense-check any quote against genuine outcomes rather than a headline figure.
The quick version
- Per square metre rates are a starting point only. Kitchens, bifold doors and groundworks can move the true figure well above the headline rate.
- Single-storey rear extensions usually fall under permitted development within the size limits, so full planning permission is often not needed, but building regulations approval is still required.
- A bigger footprint does not scale the price evenly, because the fixed costs of foundations and a roof are spread across more floor area, so small extensions often carry a higher per square metre rate than large ones.
- Fit-out choices like flooring, glazing and units often cost as much as the shell, and they are where budgets quietly overrun.
- Ground conditions are one of the biggest hidden variables. Two extensions of the same size can need very different foundation depths depending on the soil and any nearby trees.
- Ask exactly what a quote covers before comparing it against another. A shell-only price and an all-in price are not the same thing even when the square metre number looks identical.
What people actually paid
Real prices, in people's own words
- £45,000“Single storey 6.5m wide x 4m long (2022) peak inflation material costs. Included 4 keylite windows and 8ft bi fold. Gable roof. £45k”
- £51,816“Total cost £51,816 (give or take a couple of quid)”
- £80,000“We just finished a single storey. It was £2.5k per square meter plus VAT. Cost 80k in total”
- £95,000“Approx 95k, 5x3m. Included a new kitchen, downstairs WC, exterior power, knocking together upstairs bathroom & WC to make 1 room and unexpected new electrical works to most of the downstairs.”
- £95,000“Total cost £95k, of which build cost was £65k. Kitchen & utility £15k, flooring £3k, architect and planning £2k”
- £100,000“Our recent 2 storey side extension was upwards of £100k building costs.”
Genuine amounts posted publicly. We publish the price and the quote, never the person.
Why the price varies so much
The per square metre figure only ever covers the basic shell. Once you add a kitchen, a roof lantern, bifold doors, underfloor heating or a knocked-through wall with steel, the real cost climbs, and these fit-out choices are often where the biggest swings happen. Ground conditions matter more than people expect, since poor soil or nearby trees can mean deeper foundations and a bigger groundworks bill before a single wall goes up. Location plays a part too, with labour in the south east typically dearer than elsewhere in the country. Who you use matters as well. A sole trader running their own jobs tends to have lower overheads than a larger building firm with project managers and a bigger back office, and that difference shows up in the quote even for identical work. The finish level is the biggest swing of all, because a plain plastered box and a fully fitted open-plan kitchen sit at opposite ends of the range even at the same square metre size.
How to pay less
- Ask for quotes broken down into shell and fit-out so you can see where the money actually goes and trim the extras.
- Keep the design a simple rectangle. Every extra corner, roof valley or step in the wall adds labour and material.
- Source your own kitchen, tiles and flooring where the builder allows it, rather than paying a markup on their supply.
- Get the drainage and foundation approach checked early so you are not hit with a costly surprise once the ground is opened up.
- Get more than one quote and ask each builder to itemise the same list of items, so you are comparing like with like rather than one all-in figure against one shell-only figure.
- Decide your finish level before you ask for quotes. A builder pricing to a vague brief will often price high to cover themselves, and a clear spec keeps that guesswork out of the number.
Common questions
Is a per square metre price reliable?
Treat it as a rough guide, not a fixed quote. It usually covers the shell only and leaves out the kitchen, glazing, groundworks and finishes that make up a big chunk of the final bill. Use the real prices on this page to sanity-check whatever rate a builder gives you, and always ask what is and is not included in that figure.
Do I need planning permission for a rear extension?
Single-storey rear extensions often fall under permitted development as long as they stay within the depth and height limits. You will still need building regulations sign-off for the structure, insulation and drainage, which is a separate approval from planning and is not optional even when planning permission itself is not required.
Does the quote include VAT?
Most building work carries 20% VAT, so always check whether it is included before comparing prices. A quote without VAT will look cheaper on paper but cost the same once it is added, and comparing a VAT-inclusive quote against a VAT-exclusive one is a common way homeowners end up misjudging who is actually cheaper.
Why is a small extension sometimes more expensive per square metre than a big one?
Foundations, drainage connections, a new roof structure and scaffolding cost roughly the same whether the extension is small or large, so those fixed costs get spread across less floor area on a small job. A large extension does not avoid these costs, it just dilutes them, which is why the per square metre rate tends to fall as the size grows.
What is most likely to push the price above the quoted rate?
Fit-out is the usual culprit: a kitchen, bifold or sliding doors, underfloor heating and higher-spec flooring can add as much as the shell itself. Structural surprises once work starts, such as unexpected ground conditions or the need for steel to open up an existing wall, are the other common source of costs that were not in the original number.
How can I stop a low quote turning into a high final bill?
Get everything in writing before work starts, including exactly what fixtures, fittings and finish level are covered, and treat any quote that seems well below the others with more scrutiny rather than relief. A written spec with the fit-out level agreed up front is the single biggest defence against a final bill that drifts far past the original number.