Rear Extension Cost UK 2026: Prices + Calculator

A rear extension costs £1,800–£3,000 per square metre in the UK in 2026, or £36,000–£66,000 for a typical 20m² single-storey build excluding kitchens and bathrooms. The average rear extension lands around £40,000 (MyJobQuote, 2026). London and the South East add 20–50%, pushing a 20m² rear extension to £46,800–£90,000. Depth drives planning: 6m on attached houses and 8m on detached ones fall under permitted development.

How much does a rear extension cost by size?

Rear extension costs scale with the footprint — depth times the width of the house. A 3m-deep extension on a 5m-wide terrace creates 15m² for £27,000–£42,000; a full-width 6m-deep extension on the same house creates 30m² for £54,000–£84,000.

Rear extension size	Mid-range finished cost (UK, 2026)	Typical build
15m² (3m × 5m)	£27,000 – £42,000	Small rear kitchen extension
20m² (4m × 5m)	£36,000 – £56,000	Standard kitchen-diner with bi-fold doors
24m² (4m × 6m)	£43,200 – £67,200	Full-width kitchen-diner on a terrace
30m² (5m × 6m)	£54,000 – £84,000	Family kitchen-diner with utility

How much does a 3m rear extension cost?

A 3m-deep rear extension costs £27,000–£42,000 at mid-range spec for a typical 5m-wide house (15m² of new floor), or £35,100–£54,600 in London. On a 6m-wide semi-detached house the same 3m depth creates 18m² and costs £32,400–£50,400. The 3m depth is the classic kitchen extension: deep enough for a run of units plus a dining table, shallow enough to leave a real garden, and inside permitted development with room to spare — you only hit the limit at 6m (attached) or 8m (detached).

Rear Extension Cost Calculator

How much does a single storey rear extension cost?

A single storey rear extension costs £1,800–£3,300 per m² finished (FMB 2026 band; £1,800–£3,000 for a standard build). A shell-only single storey rear costs £1,300–£1,900 per m² — £26,000–£38,000 for a watertight 20m² that you fit out later. Single storey is the default rear extension: permitted development covers it to 6m or 8m deep, and the roof stays flat or pitched at eaves height. Compare the single storey extension cost guide for the full per-m² and spec breakdown.

How much does a double storey rear extension cost?

A double storey rear extension costs £2,000–£3,200 per m² of ground-floor footprint — roughly £40,000–£50,000+ for a modest two-storey build, or £108,000–£180,000 for a full-width two-storey rear (Checkatrade, 2026). Per square metre of total floor space, double storey beats single storey: the foundations, roof and access scaffolding are shared across two floors. The catch is planning — two-storey rear extensions are limited to 3m depth under permitted development, and upper-floor windows must be obscure-glazed. Most full-width double storey rears need a householder application (£258, England, 2026).

How much does a wraparound extension cost?

A wraparound extension costs £43,200–£87,400 in most of the UK and £60,000–£100,000+ in London. A wraparound combines a rear extension and a side return extension in one L-shaped build, typically adding 25–35m² of open-plan ground floor to a Victorian or Edwardian terrace. The build is cheaper than running the two extensions separately (one set of foundations, one roof, one mobilisation), but it almost always needs full planning permission: the side infill breaks the half-width permitted development limit, and most London boroughs protect terraced streets with Article 4 directions. See the side return cost guide for the infill pricing breakdown.

Do I need permission for a rear extension?

Most rear extensions need no planning permission. Permitted development covers a rear extension up to:

• 6m deep on attached houses, 8m on detached (larger home extension scheme — notify the council for prior approval; neighbours get 21 days to object)
• 4m high, or 3m within 2m of a boundary
• The 50% rule: extensions plus outbuildings cover no more than half the land around the original (1948) house
• Materials matching the existing house; eaves and ridge no higher

Full planning permission is required in conservation areas and Article 4 areas, for listed houses, for two-storey rear extensions beyond 3m deep, and whenever any limit is broken. Two-storey rear extensions under permitted development need obscure-glazed upper windows. A Lawful Development Certificate (£129) proves the build was legal — get one before you sell, not after.

What extra costs come with a rear extension?

Every rear extension carries £4,000+ of fees and statutory costs on top of the build price, and most builder quotes exclude VAT at 20%.

Extra cost	Typical fee (2026)
Architect drawings	£1,500 – £5,000
Structural engineer (calculations + drawings)	£500 – £1,500
Planning application (if needed)	£258 householder (England)
Lawful Development Certificate (if permitted development)	£129
Building control	£500 – £1,000
Party wall surveyor (per neighbour, if applicable)	£700 – £1,500
Build-over agreement (Thames Water / drains)	£350 – £600
VAT (20%) — usually excluded from builder quotes	+20% on labour and materials

What are the labour rates behind a rear extension?

Labour runs 50–60% of a rear extension build. The daily trades behind the per-m² rates (MyJobQuote, 2026):

Trade	Typical rate (2026)
Builder / groundworker	£200 – £280 per day (£20 – £35 per hour)
Bricklayer	£180 – £250 per day
Carpenter	£180 – £250 per day
Plumber	£35 – £45 per hour
Electrician (Part P registered)	£50 – £90 per hour
Plasterer	£180 – £240 per day

How long does a rear extension take?

A single-storey rear extension takes 10–16 weeks on site; a double-storey rear takes 16–24 weeks. Design, drawings and approvals add 8–16 weeks before a builder starts, so the full project runs 4–6 months for a single storey build and 6–9 months for double storey.

Stage	Typical duration
Design + structural drawings	4–8 weeks
Prior approval or planning permission	5–8 weeks (8–13 for full permission)
Builder procurement	2–4 weeks
Single-storey rear build	10–16 weeks
Double-storey rear build	16–24 weeks

Is 30k enough for a rear extension?

£30,000 builds a basic 15–18m² rear extension outside London — a 3m × 5m kitchen extension at basic spec. After drawings (£1,500–£5,000), building control (£500–£1,000), a party wall notice and 20% VAT land inside the budget, the honest all-in figure is 12–15m². In London, £30,000 buys 8–10m². The shell-only route stretches it: a watertight 15m² shell costs £19,500–£28,500, leaving the fit-out for later.

Is 50k enough for a rear extension?

£50,000 builds a 20m² rear extension at mid-range spec in most of the UK (£36,000–£56,000 build cost) with fees and VAT inside the budget. In London the same 20m² runs £46,800–£72,800, so £50,000 buys a basic finish or a smaller footprint. For the wider size and storeys comparison, see the house extension cost guide.

Frequently asked questions