A house extension costs £1,800–£3,300 per square metre in the UK in 2026, or £40,000–£140,000 for a finished build. A 20m² single-storey rear extension runs £40,000–£56,000 at mid-range spec; double-storey builds cost £2,000–£3,200 per m² of footprint but work out cheaper per square metre of floor space because foundations and roof are shared. In London and the South East, expect £2,800–£5,000 per m² — 20–40% above the UK average.
What does a house extension cost per m² in 2026?
A single-storey house extension costs £1,800–£3,300 per square metre fully finished in 2026, with the average 20m² build at £48,000 + VAT (HomeOwners Alliance). The per-m² rate falls as extensions get bigger and rises with specification. Federation of Master Builders data (March 2026) puts the single-storey average at £1,800–£3,000/m², and structural engineers' guides (SECalcs) at £1,900–£3,300/m².
How much does an extension cost by size?
Extension costs scale with footprint: a small 20m² kitchen-diner extension costs £40,000–£56,000, a 30m² family extension £60,000–£84,000, and a large 50m² open-plan rear extension £100,000–£140,000 at mid-range spec. Larger extensions cost less per square metre because fixed costs (design, access, steelwork) spread across more floor area.
| Extension size | Typical finished cost (UK, 2026) | What it gives you |
|---|---|---|
| Small (20m²) | £40,000 – £56,000 | Single-storey rear kitchen extension or extended living room |
| Medium (30m²) | £60,000 – £84,000 | Full-width rear kitchen-diner with utility |
| Large (50m²) | £100,000 – £140,000 | Large open-plan rear + side return, or double-storey over 25m² |
How much does a double storey extension cost?
A double-storey extension costs £2,000–£3,200 per square metre of ground-floor footprint — roughly £108,000–£180,000 for a typical two-storey rear build (Checkatrade, 2026). Measured per square metre of total floor space, double-storey builds are the best value in home extension work: a 5m × 6m double-storey extension creates 60m² for roughly the same structural spend as 30m² of single-storey, because the foundations, roof and access costs are shared.
What does an extension cost by specification level?
Specification moves the per-m² rate more than size does. The same 30m² footprint ranges from £36,000 as a shell to £105,000+ with premium glazing and underfloor heating.
| Spec level | Cost per m² (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, basic kitchen/bathroom if included |
| Mid-range | £1,600 – £2,000 | Good finishes, pitched or flat roof, bi-fold or slider doors, tiled floor |
| High-end | £2,000 – £3,500+ | Premium glazing, underfloor heating, structural glazing, bespoke kitchen |
The shell-only route is the least-published figure in the extension market and the one most worth knowing: a watertight 20m² shell costs £26,000–£38,000, letting you spread fit-out costs over later years.
How much more does an extension cost in London?
A house extension in London costs £2,800–£5,000 per m² — 20–40% above UK averages. Architecture for London quotes £3,000–£5,000/m² for architect-designed extensions, and Urbanist Architecture estimates a typical London extension at £40,000–£50,000 before land constraints are considered. The same 20m² that costs £40,000–£56,000 in most of the UK runs £50,000–£100,000 in the capital, driven by party wall complexity on terraces, scaffold and access costs on busy streets, and labour rates.
House Extension Cost Calculator
Set your size, spec and location for a realistic 2026 range.
What extra costs come with an extension?
Every 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) |
| 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 |
Do you need planning permission for an extension?
Many single-storey rear extensions fall under permitted development, which means no planning application if the rules are met:
- Rear extension: max 4m high (3m within 2m of a boundary); up to 6m deep for attached houses, 8m detached (larger sizes need prior approval from the council)
- Side extensions: single-storey only, max 4m high, max half the width of the original house
- The 50% rule: extensions plus all outbuildings cover no more than half the land around the original house (as it stood in 1948)
- Materials must match the existing house; eaves and ridge must not exceed it
You need full planning permission when: the design breaks any permitted development limit, the property is in a conservation area or an Article 4 area, the house is listed, or permitted development rights have been removed. A Lawful Development Certificate (£129) proves your permitted development build was legal — get one even when no permission is needed, or you will answer questions when selling.
Is it cheaper to extend or move house?
For most families, extending is cheaper than moving. Moving from a £400,000 house costs £18,000–£30,000 in stamp duty, estate agent fees, legal fees and removals before adding a single square metre — and then you buy the extra space at full market price. A 20m² extension creating the same space costs £40,000–£56,000 and increases your property value by roughly 20% (Nationwide research). The maths flips only when the planned extension would cost more than about 25% of the property value, or where the garden is too small for the space you need.
Is 30k enough for an extension?
Yes — for a basic single-storey build outside London, with nothing left over. At £1,500–£1,600/m², £30,000 builds an 18–20m² rear extension to a basic finish. The reality check: fees (£2,000–£4,000), VAT (20% on most quotes) and even a modest kitchen (£5,000+) must fit inside the same £30,000, so the honest answer is a 15m² build with a £30,000 all-in budget in most of the UK, or 8–10m² in London. A shell-only route stretches it furthest: £26,000–£38,000 for a watertight 20m² that you fit out later.
How long does a house extension take?
| Stage | Typical duration |
|---|---|
| Design + structural drawings | 4–8 weeks |
| Permitted development certificate or planning permission | 5–8 weeks (8–13 for full permission) |
| Builder procurement | 2–4 weeks |
| Single-storey build | 10–16 weeks |
| Double-storey build | 16–24 weeks |
Budget 4–6 months from first sketch to keys for a single-storey extension. The build itself disrupts the back of the house for its full duration — plan a temporary kitchen if the extension replaces one.
What adds more value, an extension or a loft conversion?
Both add roughly 20% to property value when they create a double bedroom plus bathroom (Nationwide). Extensions cost more in total (£40,000–£140,000 vs £25,000–£100,000 for loft conversions) but create the ground-floor space buyers want most in 2026: open-plan kitchen-diners. Choose by what the house lacks — no ground-floor living space means an extension; no bedrooms means a loft. Many 2026 projects do a garage conversion first at £10,000–£20,000, then reassess.
Frequently asked questions
Is 30k enough for an extension?
Yes for a basic 15–20m² single-storey build outside London — after fees and VAT, it is tight. London buys 8–10m².
How much is a 20 square metre extension?
£40,000–£56,000 finished at mid-range spec in most of the UK; £26,000–£38,000 shell-only; £50,000–£100,000 in London.
Is it cheaper to extend or move house?
Usually — moving costs £18,000–£30,000 in fees alone on a £400k house, then you buy space at full price. Extending adds it for £40k–£56k and adds value.
What is the 50% rule for house extensions?
Extensions and outbuildings together must cover no more than 50% of the land around the original (1948) house. Break it and you need planning permission.
How much does a double storey extension cost?
£2,000–£3,200 per m² of footprint, £108,000–£180,000 typical. Better value per m² of floor space than single-storey.
How much does a house extension cost in London?
£2,800–£5,000 per m², 20–40% above UK averages. A typical 30m² London extension: £84,000–£150,000.
How high can a single storey extension be without planning permission?
Up to 4m high (3m within 2m of a boundary), 6m deep attached or 8m detached under the larger extension scheme with prior approval.
What adds more value, an extension or a loft conversion?
Both around 20% for a bedroom plus bathroom. Extensions create the kitchen-diner space buyers want; lofts create bedrooms cheaper.
Ready for real quotes?
Tell us where to send them — we'll match your project with a vetted local specialist and email your free, no-obligation quotes.
No spam, no call centres — just your quotes.