How Much Does a Single-Storey Extension Cost?

A single-storey extension is one of the most popular ways UK homeowners add real living space without moving house — but the cost range you'll see quoted is genuinely wide, and for good reason. Size, specification, and whether you're extending a simple room or a full kitchen-diner all move the number significantly. As a starting point: a typical medium-sized single-storey extension (roughly 15–25m²) at standard specification, including fit-out, lands somewhere in the £35,000–£75,000 range.

Typical UK single-storey extension costs

These are broad, genuinely typical ranges for a complete project (structure plus fit-out) as of writing — not a quote for any specific project. Actual cost depends heavily on footprint, specification, access, and existing ground conditions.

• Small extension (~10–15m²): roughly £18,000–£45,000
• Medium extension (~15–25m²): roughly £35,000–£75,000
• Large extension (25m²+): roughly £60,000–£120,000+

Expressed per square metre, that's roughly £1,800–£2,500/m² for the shell alone (structure, roof, basic weatherproofing) and £2,500–£3,500+/m² including a full fit-out — flooring, kitchen or bathroom fittings if relevant, decoration, and finishing. Shell-only and fully-fitted figures aren't directly comparable, so always confirm which one a quote covers.

A worked example

Take a fairly typical case: a 4m x 5m (20m²) single-storey rear extension to create a larger kitchen-diner, standard specification, no unusual ground conditions. At £2,500–£3,500/m² fully fitted, that's roughly £50,000–£70,000 total — sitting toward the upper-medium band above. A simpler reception room or home office of the same footprint, without the plumbing and kitchen fit-out, would typically come in £10,000–£15,000 lower, since a large share of that gap is the kitchen units, appliances and services rather than the structure itself.

Roughly where the money goes

For a standard-specification extension, the broad stages typically break down something like this — actual proportions shift with specification and site conditions:

• Groundwork and foundations: roughly 15–20% of the build
• Structure, walls and roof: roughly 30–35%
• Windows, doors and glazing: roughly 10–15%, more if large sliding/bi-fold doors are specified
• First-fix plumbing and electrical: roughly 10%
• Plastering, second-fix and finishing: roughly 20–25%

See Extension Labour Costs Explained for what each of these stages actually involves.

What actually drives the price

Size and footprint shape

Larger extensions generally cost less per square metre than small ones, since fixed costs (scaffolding, site setup, professional fees) spread across more space — but a complex or irregular footprint can offset that saving.

Foundation and ground conditions

Standard ground conditions keep foundation costs predictable. Sloping sites, poor soil, or proximity to trees and drains can require deeper or more complex foundations, adding real cost before the visible structure even starts.

Roof type

A simple flat or lean-to roof is generally the most cost-effective. Matching an existing pitched roof, or adding rooflights and lantern windows, adds both material and labour cost.

Glazing

Large sliding or bi-fold doors and extensive glazing look striking but cost considerably more than standard windows and a single door — both in the units themselves and the structural openings needed to support them.

What the extension is used for

An extended kitchen-diner needs plumbing, electrics, and often a full kitchen fit-out on top of the build — see our dedicated kitchen extension cost guide if that's your project. A simple additional reception room or home office is comparatively straightforward.

Do you need planning permission?

Many single-storey rear extensions fall within Permitted Development, subject to conditions. As a general guide (always confirm current specifics with your local planning authority, since exceptions apply): a standard single-storey rear extension can extend up to 4m for a detached house or 3m for a semi-detached/terraced house under standard Permitted Development, or further — 8m and 6m respectively — under the Larger Home Extension prior-approval route, which involves notifying neighbours but not a full planning application. Maximum height is generally 4m, reducing to 3m for eaves within 2m of a boundary. See our full guide on Permitted Development rights for extensions for the complete picture, including what takes a project outside these limits.

Party wall matters

If your extension involves work on, or near, a shared wall or boundary with a neighbouring property — common with semi-detached and terraced houses — you may need to serve a Party Wall Act notice before work starts. See our guide on Party Wall Agreements for when this applies and how much notice is required; it's worth checking early, since the notice periods (one to two months) can affect your start date if left to the last minute.

What you can and can't do yourself

Groundwork, structural work, roofing and anything requiring building control sign-off should be done by qualified trades — this isn't a project where DIY meaningfully reduces cost without real risk. Where homeowners can genuinely save money is in the finishing stages: decoration, and sometimes flooring, once the structural shell is complete and signed off. Clearing the site and garden before work starts is also a straightforward way to save on labour time without touching anything structural.

Labour vs materials

See our companion guide, Extension Labour Costs Explained, for how the trade cost breaks down against materials on a typical build.

How long it takes

Budget and timeline go hand in hand — see How Long Does a Single-Storey Extension Take? for a realistic build programme.

Avoiding the common pitfalls

See our common extension mistakes guide for where extension budgets typically go wrong, and our general renovation budgeting guide for contingency planning that applies to any project this size.

Extension vs moving

If you're weighing up whether extending is actually the right financial move compared with moving house, see our dedicated comparison: Extension vs Moving House.

Getting an accurate quote

Because ground conditions and design vary so much between properties, the only reliable way to get an accurate figure is a proper site visit and a detailed, itemised quote.