How much does a house extension cost in the UK?
The average cost of a single-storey house extension in the UK is £45,000–£75,000 for a standard build of around 20–30 m². This includes the builder's labour and materials but excludes VAT (20%), architect fees, structural engineer fees and interior fit-out.
At a per-square-metre level, expect to pay £1,800–£2,500 per m² for a standard spec extension across most of the UK. In London and the South East, prices are typically 25–40% higher: £2,200–£3,200 per m².
Important: these figures are for the build cost only. Add 20% VAT, £3,000–£8,000 for architect and structural engineer fees, and your interior fit-out costs on top. A 30 m² kitchen extension all-in often lands between £60,000 and £95,000 by the time the kitchen is installed and the room is decorated.
Costs by extension type (2026)
| Extension type | Typical build cost (UK average) | South of England |
|---|---|---|
| Single-storey rear (20 m²) | £36,000–£50,000 | £44,000–£65,000 |
| Single-storey rear (30 m²) | £54,000–£75,000 | £66,000–£96,000 |
| Side return extension (15 m²) | £27,000–£40,000 | £33,000–£52,000 |
| Double-storey rear (40 m²) | £60,000–£100,000 | £75,000–£130,000 |
| Wrap-around extension | £90,000–£160,000 | £110,000–£200,000+ |
Double-storey extensions cost less per m² than single-storey because the foundation, scaffold and roof are shared across two floors.
What is included in the build cost?
A typical builder's quote for an extension covers:
- Foundations (strip or raft)
- Brickwork and blockwork (cavity wall construction)
- Structural steel or concrete lintels
- Roof structure and covering
- External windows and doors (to standard spec)
- First fix plumbing and electrical routes
- Plasterboard and plastering
- Screed or structural floor
What is NOT included
These costs are almost always separate and need to be budgeted for in addition:
- VAT (20%) — applies to all labour and most materials
- Architect drawings — £2,000–£6,000 for planning drawings; £4,000–£10,000 for full service
- Structural engineer — £800–£2,500 for foundation and steel design
- Planning permission application — £258 (England, 2026) if required
- Building Regulations inspector — £600–£1,200 depending on local authority
- Party wall surveyor — £700–£1,500 per neighbour if required
- Kitchen or bathroom fit-out — hugely variable, from £5,000 to £40,000+
- Flooring and decoration — £2,000–£8,000 for a typical extension
What affects the cost most?
Size
The bigger the extension, the higher the total cost. However, cost per m² usually falls slightly as size increases because fixed costs (scaffold, foundations, roof) are spread over more floor area.
Specification
Bifold or sliding doors, roof lanterns, polished concrete floors and premium glazing all add significantly to cost. A standard spec bi-fold door opening costs £3,000–£6,000 more than a standard French door in the same opening.
Structural complexity
Opening up a wall between the extension and the main house requires a structural steel beam. Each beam typically adds £1,500–£4,000 to the job (engineer fees, steel supply, temporary support and bricklayer/builder time to install). Large open-plan spans with multiple beams push costs up significantly.
Matching existing brickwork
If you want the extension exterior to match the existing house brickwork — which most people do — this adds time and cost. Sourcing a matching brick, and potentially reclaimed bricks for older properties, adds to the material cost. See our guide to matching brickwork for more detail.
Ground conditions
Poor ground — made-up ground, high water table, clay that moves seasonally, or proximity to trees — can significantly increase foundation costs. Standard strip foundations cost £4,000–£8,000 for a typical extension; pile foundations for difficult ground can cost £15,000–£30,000 more.
Location
Labour costs vary significantly across the UK. London and the South East are the most expensive regions. The same extension that costs £50,000 to build in the Midlands may cost £65,000–£70,000 in Hampshire or Surrey.
Does an extension add value?
Yes — but not always enough to cover the full cost of building it. A well-designed single-storey kitchen-diner extension typically adds 5–8% to a property's value. A double-storey extension adding a bedroom and bathroom can add 10–20%, particularly if it takes the property from three to four bedrooms.
The best value extensions are those that add living space that buyers want — a larger kitchen, an extra bedroom, or a home office — in areas where property prices are high enough that the uplift covers the build cost.
How long does a house extension take?
| Stage | Typical duration |
|---|---|
| Design + planning drawings | 4–8 weeks |
| Planning permission decision (if needed) | 8 weeks |
| Building Regulations approval | 5–8 weeks (can run in parallel) |
| Contractor procurement + mobilisation | 4–12 weeks |
| Construction (single-storey, 25 m²) | 10–16 weeks |
| Interior fit-out (kitchen, flooring, decorating) | 4–8 weeks |
Total from first drawing to moving back in: typically 9–18 months for a single-storey extension when you include planning and fit-out. If your extension falls within permitted development (no planning application needed), the programme is typically 6–12 months.
Frequently asked questions
- Do I need an architect for a house extension?
- Not legally — but it is usually worth it. An architect produces planning drawings and can help you get the most out of the space. For complex or large extensions, a full architectural service (design through to construction monitoring) is worth the fee. For a simple single-storey rear extension, some builders work from a structural engineer's drawings alone, or you can use a technologist or architectural designer at lower cost than a fully qualified architect.
- Do I need a structural engineer for an extension?
- Yes, in almost every case. Building Regulations require structural calculations for foundations, lintels and any new steel beams. The structural engineer produces these drawings and calculations. Budget £800–£2,500. Your architect or builder can recommend one.
- Can I manage the extension myself without a main contractor?
- Yes — self-managing trades (bricklayer, roofer, electrician, plumber) directly can save 10–20% on a main contractor's margin. However, it requires significant time, organisation and knowledge of the building process. Programme delays caused by one trade affect all others. Most people find it worthwhile for extensions over £80,000 where the saving is meaningful.
- What is the cheapest type of house extension?
- A single-storey rear extension with a flat or lean-to roof, standard bi-fold or French doors, and rendered blockwork (rather than matching brickwork) is typically the most cost-effective. Flat roofs cost less than pitched roofs. Simple rectangular plans cost less than L-shapes or complex rooflines.