A single storey extension in 2026 typically costs £1,800–£3,800 per square metre depending on specification, which for a fairly ordinary rear or side extension works out at roughly £40,000–£90,000 built — more for a large wraparound job, more again if you're in London or the South East. That per-m² figure is the number every builder's quote leads with, and it's the one most homeowners fixate on.
But the number that actually decides whether the project runs smoothly, in my experience, isn't the build cost — it's whether you've correctly worked out if you need planning permission at all, and whether the extension touches ground or a wall your neighbour has a legal interest in. Both are cheap to get right before you start and expensive to get wrong once the foundations are in.
I'm a building surveyor, not an extension contractor, so what follows is 2026 pricing cross-checked against current UK cost guides, what actually moves the per-m² number, and the planning and party wall groundwork I'd want settled before anyone digs a trench.
The short answer
Figures below were checked on 20 August 2026 against 2026 cost guides from Checkatrade, MyJobQuote, BookaBuilderUK and other current UK extension-pricing guides, cross-referenced against each other. As with the rest of this series, London and the South East — my own patch included — trend well above the national figures quoted here, more so on extensions than most trades because of higher labour rates, tighter access and a greater chance of needing a party wall award.
| Scenario | Typical cost (2026) | What's included |
|---|---|---|
| Small side or utility extension (10–15m²) | £22,000–£40,000 | Basic-to-mid spec, standard windows and doors, minimal structural work |
| Typical rear or kitchen extension (15–25m²) | £40,000–£75,000 | Bi-fold or sliding doors, underfloor heating, one or two structural openings |
| Larger single-storey extension (25–40m²) | £70,000–£130,000 | Wraparound or open-plan, glazed roof lantern, multiple structural steels, higher-spec finish |
| London & South East | Add 20–40% to the above | Higher labour rates, scaffolding and access costs, greater likelihood of a party wall award |
Those figures are build cost — labour, materials, a standard fit-out, and the contractor's own VAT-inclusive price. They don't include the professional and legal fees covered further down, which typically add another 10–20% on top depending on whether you need planning permission, a structural engineer, and a party wall surveyor.
What drives the price
Specification. Basic joinery and standard aluminium bi-folds sit at the bottom of the per-m² range; a glazed roof lantern, engineered timber flooring and a high-end kitchen fit-out can push the same footprint from £2,000/m² to £3,500/m² without changing the structural scope at all.
Structural complexity. Open-plan designs that remove most of the rear wall need a steel beam, sometimes two, spanning further than a standard lintel. Every extra steel, and every metre it has to span, adds to both the structural engineer's fee and the build cost.
Roof type. A simple flat or warm-flat roof is the cheapest option; a pitched roof matching the existing house costs more in materials and labour; a glazed lantern or full glass roof, increasingly the default on kitchen extensions, is the single biggest line item after the structural steels.
Ground conditions. Clay soil, nearby trees, made-up ground or a sloping site all mean deeper or more complex foundations than a straightforward trench-fill strip footing, and none of it is visible until someone digs a trial pit.
Site access. A narrow side passage that rules out a machine, or a build that has to go over next door's garden with permission, adds labour time that a wide-open plot never sees.
Region. London and the South East commonly run 20–40% above the national figures quoted here, driven by labour rates, parking and access restrictions, and a much higher likelihood of needing a formal party wall agreement given how close most terraced and semi-detached houses sit to their neighbours.
Planning permission or permitted development?
Most single-storey rear and side extensions on houses in England are built under permitted development (PD) rather than a full planning application, which is the main reason the planning fee doesn't appear in most builders' quotes at all. The standard PD limit lets you extend the rear wall by up to 4 metres on a detached house, or 3 metres on a semi-detached or terraced house, without needing permission — subject to a maximum height of 4 metres, an eaves height of no more than 3 metres where the extension comes within 2 metres of a boundary, and the extension not being taller than the original house.
Want to go further, up to 8 metres on a detached house or 6 metres on any other house, and there's a halfway route: prior approval, sometimes called the neighbour consultation scheme. You notify your local planning authority, who in turn notify the adjoining neighbours; if nobody objects within 21 days the extension is treated as approved, and if someone does, a planning officer decides. The notification fee is modest and set by your council, typically a few hundred pounds rather than the full application fee — worth confirming directly with them, since the amount and how it's described varies by authority.
Where PD doesn't apply at all — flats, listed buildings, land in a conservation area or other designated area, or a design that exceeds even the larger PD limits — you need a full householder planning application. That currently costs £548 in England as of the April 2026 fee indexation, up from £528 when the fee was more than doubled in April 2025, and £258 before that. If you've built under PD and simply want it on record for a future buyer or lender, a Lawful Development Certificate costs half the planning fee, around £274, and is one of the cheapest pieces of paperwork you can buy relative to the argument it settles at sale.
The party wall step most quotes skip
This is the one that catches people out, because it's a legal obligation, not a council process, and no builder's quote is going to raise it for you. The Party Wall etc. Act 1996 applies to a single-storey extension in three common situations: excavating new foundations within 3 metres of a neighbour's building where your foundations will go deeper than theirs (or within 6 metres, on a steeper 45-degree test), building a new wall astride or right up to the boundary line, and cutting into or otherwise working on a wall you already share with next door.
Notice periods differ by which of those applies — one calendar month for excavation and line-of-junction notices, two calendar months for work on an existing shared wall — and none of it can be skipped by mutual goodwill; it has to be served in writing. Where you and your neighbour agree to use one surveyor between you, that typically costs £900–£2,000; if either side wants their own, the combined bill runs £1,800–£5,400, and as the building owner carrying out the works, you normally pay both surveyors' reasonable fees, including the one your neighbour chose. Build the notice period into your programme from the outset — it's the single most common cause of an extension start date slipping by a month or two.
What I'd watch for in a quote
- Whether the per-m² figure is a true all-in price or a shell-only figure that excludes kitchen, flooring, decorating and fees — ask what's specifically excluded before comparing two quotes on price alone.
- Structural engineer's calculations and Building Control sign-off priced in or left for you to arrange separately — typically another £500–£1,500, covered in full in structural engineer cost.
- A ground investigation or trial pit done, or at least discussed, before the price is fixed rather than after the trench is dug and a problem is found.
- A genuine contingency, typically 10–15% on top of the headline figure, rather than a quote that assumes nothing unexpected turns up.
- Who's responsible for party wall compliance — some contractors will flag it, most won't, and it's your legal obligation as the building owner regardless of who forgot to mention it.
Questions I get asked
How much does a single storey extension cost per m² in 2026? Expect £1,800–£2,400 per m² for a basic specification, £2,400–£3,000 per m² for a mid-range build with bi-fold doors and underfloor heating, and £3,000–£3,800 per m² or more for a high-end, glazing-heavy design. London and the South East typically run 20–40% above those figures.
Do I need planning permission for a single storey extension? Often not. Permitted development allows up to 4 metres from the original rear wall on a detached house, or 3 metres on a semi-detached or terraced house, without permission, subject to height and boundary limits. Larger rear extensions, up to 8 metres detached or 6 metres for other houses, are possible through the prior approval route. Flats, listed buildings and land in a conservation area don't get the same rights, and exceeding the limits without going through prior approval means a full householder application, currently £548 in England.
Do I need a party wall agreement for a single storey extension? Usually, if the extension sits close to a neighbour's boundary or building. Excavating within 3 metres of their building and deeper than their foundations, building astride or up to the boundary line, or cutting into a shared wall all trigger the Party Wall etc. Act 1996. One agreed surveyor typically costs £900–£2,000; two separate surveyors run £1,800–£5,400 combined, usually paid by you as the building owner.
How long does a single storey extension take to build? A typical 15–25m² rear or side extension usually takes 10–16 weeks on site, longer for a larger or structurally complex build. That excludes the weeks or months beforehand for design, planning or prior approval, Building Regulations approval, and any statutory party wall notice period before digging can legally start.
If you're weighing up a single storey extension and want an honest read on structural feasibility, whether permitted development genuinely covers what you're planning, or how exposed you are to a party wall notice, before you commission drawings or sign a builder's contract — that's exactly the kind of instruction I take on alongside full building survey work. I've covered what a structural engineer's own fee is likely to add in structural engineer cost. Call 07946 618203 or get in touch and I'll give you a straight answer on feasibility before you commit.
Small print. This guide is general information, not advice on a specific property, and it doesn’t replace a proper survey, structural design and written quote for your own home. Prices were researched and correct to the best of my knowledge on 20 August 2026; costs, planning fees and local authority charges move and vary by region and property. Always get at least three written quotes, and confirm your permitted development and party wall position in writing before work starts.