Single Storey Extension Cost UK 2026: Prices + Calculator

A single storey extension costs £1,800–£3,300 per square metre finished in the UK in 2026, or £36,000–£60,000 for a typical 20m² build. A 12m² extension runs £21,600–£33,600 at mid-range spec; a 30m² extension £54,000–£84,000. London and the South East add 20–30%, pushing the per-m² rate to £2,500–£4,000. Every figure below is cross-checked against FMB, Checkatrade and structural engineers' 2026 price guides.

What does a single storey extension cost per m² in 2026?

A single storey extension costs £1,800–£3,300 per square metre for a standard finished build (FMB, 2026). The rate moves with specification more than with size: a watertight shell costs £1,300–£1,900 per m², while a high-end build with premium glazing and underfloor heating reaches £2,000–£3,500 per m².

Specification	Cost per m² (UK, 2026)	What you get
Shell only	£1,300 – £1,900	Watertight structure: foundations, walls, roof, doors, windows, first-fix electrics. You fit out later.
Basic	£1,200 – £1,600	Standard materials, flat roof, plastered finish, basic fittings
Mid-range	£1,800 – £2,800	Bi-fold or sliding doors, rooflights, tiled floor, standard kitchen allowance
High-end	£2,000 – £3,500+	Premium glazing, structural glazing, underfloor heating, bespoke kitchen

Some 2026 guides quote tighter mid-range bands of £1,500–£2,200 per m² for single storey builds (Dakea, 2026); the ranges on this page use the wider overlapping band from FMB and structural engineers' guides, because real builder quotes spread across both.

How much does a single storey extension cost by size?

Single storey extension costs scale with footprint, and the per-m² rate falls as the extension grows because fixed costs (design, access, steelwork) spread across more floor area. A 12m² build costs £21,600–£33,600 at mid-range spec; a 40m² open-plan build costs £72,000–£112,000.

Extension size	Mid-range finished cost (UK, 2026)	Typical build
12m² (3m × 4m)	£21,600 – £33,600	Small rear kitchen extension or garden room
20m² (4m × 5m)	£36,000 – £56,000	Standard kitchen-diner with bifold doors
30m² (5m × 6m)	£54,000 – £84,000	Full-width family kitchen-diner with utility
40m² (5m × 8m)	£72,000 – £112,000	Large open-plan living space, permitted development maximum depth

How much does a 3m by 4m extension cost?

A 3m by 4m single storey extension (12m²) costs £21,600–£33,600 at mid-range spec outside London, or £28,100–£43,700 in London after the 20–30% regional premium. A basic 3m × 4m build runs £14,400–£19,200; a high-end 12m² build £24,000–£42,000. The 3m depth matters for planning: a 4m-deep rear extension on an attached house sits inside the 6m permitted development limit, so most 3m × 4m builds need no planning application — just a prior approval notification and a Lawful Development Certificate (£129) if you want proof for resale.

Single Storey Extension Cost Calculator

What does a single storey extension cost in London?

A single storey extension in London costs £2,500–£4,000+ per m² — 20–30% above UK averages (FMB, 2026). The same 20m² build that costs £36,000–£56,000 in most of the UK runs £46,800–£72,800 in the capital. Terraced sites drive the premium: party wall agreements (£700–£1,500 per neighbour), restricted access, scaffold over pavements and higher labour rates. A typical 30m² London single storey extension costs £70,200–£109,200.

Do you need planning permission for a single storey extension?

Most single storey rear extensions fall under permitted development — no planning application, provided the limits are met:

• Rear extension depth: 6m for attached houses, 8m for detached (larger home extension scheme, with prior approval notification to the council)
• Height: max 4m for a rear extension, 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 must match the existing house; eaves and ridge must not exceed it

Full planning permission is required in conservation areas, Article 4 areas (common across London boroughs), for listed houses, and whenever any limit is broken. The householder application fee is £258 in England (2026). A Lawful Development Certificate (£129) proves a permitted development build was legal — get one before you sell, not after.

What extra costs come with a single storey extension?

Every single storey 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, terraced and semi-detached)	£700 – £1,500
Build-over agreement (Thames Water / drains)	£350 – £600
VAT (20%) — usually excluded from builder quotes	+20% on labour and materials

How long does a single storey extension take?

A single storey extension takes 10–16 weeks on site. Design, drawings and approvals add 8–16 weeks before a builder starts, so the full project runs 4–6 months from first sketch to completion.

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 build on site	10–16 weeks

Is 30k enough for a single storey extension?

£30,000 builds a basic 15–18m² single storey extension outside London, with nothing left for fees or fit-out. Once drawings (£1,500–£5,000), building control (£500–£1,000) and 20% VAT land inside the same budget, the honest figure is a 12–15m² build. In London, £30,000 buys 8–10m². The shell-only route stretches a £30,000 budget furthest: a watertight 20m² shell costs £26,000–£38,000, and you fit the kitchen and floor later.

Is 50k enough for a single storey extension?

£50,000 builds a 20m² single storey extension at mid-range spec in most of the UK (£36,000–£56,000 build cost) with roughly £5,000 left for fees and VAT. In London, £50,000 covers a basic 12–15m² build only; the same 20m² mid-range build runs £46,800–£72,800. For a wider comparison of sizes and storeys, see the full house extension cost guide or the rear extension cost guide for depth-specific builds.

Is it cheaper to buy a bigger house or extend?

Extending is cheaper. Moving from a £400,000 house costs £18,000–£30,000 in stamp duty, estate agent and legal fees before you gain a single square metre, while a 20m² single storey extension creating the same space costs £36,000–£56,000 and adds roughly 20% to property value (Nationwide research). Extending loses only when the build would cost more than 25% of the property value, or when the garden is too small to give up.

What does a builder's single storey extension quote include?

Builder quotes for single storey extensions exclude more than they include — check each line before comparing quotes.

Usually included	Usually excluded
Foundations, brickwork, roof, glazing	VAT at 20% (add it yourself)
First-fix electrics and plumbing	Kitchen, appliances, flooring
Plastering and decoration-ready finish	Architect, structural engineer, building control fees
Scaffold, skip, site clearance	Party wall surveyor, build-over agreement

Frequently asked questions