Single Storey Extension Cost UK 2026 + Calculator

Updated August 2026 · Prices researched from FMB, Checkatrade, HomeOwners Alliance and structural engineers' 2026 guides

A single storey extension costs £1,500–£3,300 per square metre in the UK in 2026 — £40,000–£56,000 for the typical 20m² rear build. Shell-only construction drops to £1,300–£1,900/m², high-end finishes reach £3,500/m², and London adds 20–40%. The single storey rear extension is the UK's most-built extension type: one level, one roof, no stairwell, and usually inside permitted development.

How much does a single storey extension cost per m²?

Single storey extension rates in 2026: £1,500–£3,300 per m² fully finished for the mainstream build, with FMB's March 2026 guide at £1,800–£3,000/m² and SECalcs at £1,900–£3,300/m². The specification spread matters more than size: the same 20m² footprint spans £26,000 shell-only to £70,000 high-end.

How much is a 20 square metre extension?

A 20m² single storey extension costs £40,000–£56,000 finished at mid-range spec in most of the UK, £26,000–£38,000 shell-only, and £50,000–£100,000 in London. The HomeOwners Alliance puts the average 20m² single-storey build at £48,000 + VAT. What 20m² actually buys: a kitchen extension replacing a small existing kitchen and eating area, or a living-room extension — enough for a 4-5m table zone or a generous sofa arrangement, not a full open-plan ground floor (that's the 30-50m² tier in our main extension cost guide).

How much does a rear extension cost?

A rear single storey extension costs £40,000–£66,000 for a typical 20m² build (SECalcs, 2026: "£1,900–£3,300/m², a 20m² rear extension £40,000–£66,000"). Rear extensions run full-width or partial-width off the back of the house, usually replacing or wrapping existing back rooms. Deep rear extensions (6-8m) need prior approval under the larger home extension scheme; shallower ones sit inside standard permitted development.

How much does a side return extension cost?

A side return extension costs £18,000–£45,000 for the typical 8-15m² infill beside a Victorian or Edwardian back room — the cheapest extension type per project because the footprint is small, though per-m² rates run higher (£2,000–£3,000/m²) on short runs of wall and roof. Combined side-return-plus-rear builds on terraces total £45,000–£70,000 and create the full-width kitchen-diner that defines the renovated Victorian terrace. Watch the light: a 3m-deep side return needs a rooflight or lantern to stop the middle of the ground floor going gloomy.

Single Storey Extension Cost Calculator

What extra costs come with a single storey extension?

Foundations deserve their own line: strip or trench-fill to frost depth (1m minimum, deeper on London clay), inspected by building control before the pour. Trees, drains or made ground push the dig deeper and the bill higher — a soil-contaminated or tree-protected site moves foundations from £3,000 to £10,000+.

How high can a single storey extension be without planning permission?

Permitted development allows a single storey rear extension up to 4m high (3m within 2m of a boundary), extending up to 6m beyond the rear wall of an attached house or 8m detached under the larger home extension scheme — the larger depths need prior approval (neighbour consultation) before building. Side extensions: single-storey, max 4m high, max half the width of the original house. Eaves and ridge must not exceed the existing house, materials must match, and the 50% rule caps total built coverage of the garden. Break any limit and it's a full planning application (£258) — or check whether your council has an Article 4 direction removing these rights entirely (widespread in London boroughs).

What is the difference between a rear extension and an orangery?

Valuers and buyers treat solid rear extensions as superior: no thermal-bridge problems at the wall-roof junction, no summer overheating, and full EPC treatment. Orangeries suit period homes where a full solid extension would overwhelm the original elevation.

Is a single storey extension worth it?

A single storey extension creating a genuine open-plan kitchen-diner adds roughly 10–20% to property value against a £40,000–£84,000 build — and kitchen-diner space is the single strongest buyer-demand signal in 2026 listings. The uplift concentrates in the flow created: extensions that merge kitchen, dining and living zones with good light outperform pure utility add-ons. Compare alternatives: a loft conversion adds similar percentage for a similar spend (bedrooms vs living space), a garage conversion adds ~10% for half the money, and a garden room adds 5-8% with zero structural work to the house.

How long does a single storey extension take to build?

Total: 4–6 months from sketch to keys for the typical single storey build. Weather-tight at week 8 is the milestone that matters — everything after it is interior work and unaffected by rain.

Does a single storey extension need foundations?

Yes — always. Strip or trench-fill foundations to frost depth (1m minimum; London clay routinely demands 1.2–1.5m), plus steel beams where the extension opens into the original house. Building control inspects the open trench before any concrete goes in; an uninspected pour gets dug out. Foundation cost runs £3,000–£6,000 in the finished build, with tree proximity, drains and made ground as the three cost escalators — all discoverable with a £150–£300 trial hole before you commit.

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