A house extension in the UK typically costs £1,800 to £3,300 per m² in 2026, before VAT. A 20m² single storey extension usually costs around £40,000–£56,000, and a 30m² extension around £60,000–£84,000. A double storey extension costs more overall, often £108,000–£180,000 for 60m², but is usually cheaper per square metre. London and the South East are typically 20–35% more expensive.
This guide explains how much each type of extension costs, what affects the price, the fees to budget for, and how to get an accurate price for your project.
House Extension Cost UK 2026: Quick Summary
| Type of extension | Typical cost per m² (ex VAT) | Example total |
|---|---|---|
| Single storey (standard) | £1,800 – £2,500 | 20m²: £40,000 – £56,000 |
| Single storey (high spec) | £2,500 – £3,300 | 30m²: £75,000 – £110,000 |
| Double storey | £2,000 – £3,200 | 60m²: £108,000 – £180,000 |
| Side return | £2,800 – £3,800 | Depends on width and glazing |
| Kitchen extension | £2,200 – £3,300 + kitchen | 30m²: £66,000 – £99,000 |
Figures are based on 2026 UK cost guides from Homebuilding & Renovating and Checkatrade (sources listed at the end). Prices exclude VAT unless stated and vary by location, size, design and specification. Use them as a guide, not a quote.
How Much Does a Single Storey Extension Cost?
A single storey extension typically costs £1,800 to £3,300 per m², or around £40,000–£56,000 for 20m² and £60,000–£84,000 for 30m². Single storey rear extensions are the most common type in the UK, often used to create a larger kitchen, dining or living space.
| Size | Typical cost (ex VAT) |
|---|---|
| 20m² | £40,000 – £56,000 |
| 30m² | £60,000 – £84,000 |
| 30m² (premium finish) | £75,000 – £110,000 |
| 50m² | £100,000 – £140,000 |
The price per m² depends on the specification. As a guide, a standard finish is around £2,200–£2,500 per m², a good finish £2,500–£2,900 per m², and an excellent finish £2,900–£3,300 per m².
How Much Does a Double Storey Extension Cost?
A double storey extension typically costs £2,000 to £3,200 per m², or around £108,000–£180,000 for a 60m² extension. Although the total is higher, a double storey extension is usually better value per square metre. The foundations and roof, which are some of the most expensive parts of any extension, are shared across two floors.
As a rule of thumb, a two storey extension can give you around twice the space of a single storey extension for roughly 50–70% more cost. Adding a first floor over an existing single storey extension typically costs around £45,000–£75,000 for 25m².
How Much Does a Kitchen Extension Cost?
A 30m² single storey kitchen extension typically costs around £66,000–£99,000 plus VAT, including the new kitchen. Compared with a standard extension, a kitchen usually adds around £15,000–£35,000, depending on the units, worktops and appliances. Extra drainage, plumbing, electrics and glazing such as bifold or sliding doors also add to the cost.
How Much Does a Side Return Extension Cost?
A side return extension typically costs around £2,800 to £3,800 per m². It fills the narrow passage along the side of many terraced and Victorian houses. Because the space is small, the fixed costs, such as structural steel and glazed roofs, are spread over fewer square metres, so the price per m² is higher. A wraparound extension combines a side return and a rear extension in an L-shape, and usually costs the most overall.
What Affects the Cost of a House Extension?
- Size: bigger extensions cost more in total, but usually less per m².
- Single or double storey: double storey costs more overall, but less per m².
- Specification: finishes, glazing, flooring and the kitchen have a big effect.
- Glazing: bifold doors, sliding doors and roof lanterns add significant cost.
- Ground conditions: poor soil, trees or drains nearby can mean deeper or special foundations.
- Structural work: knocking through to the existing house usually needs steel beams.
- Location: London and the South East are typically 20–35% more expensive.
- Access: restricted access, such as a terraced house with no side access, adds labour time.
Extra Costs and Fees to Budget For
| Item | Typical cost |
|---|---|
| Architect or design fees | £4,500 – £6,500, or 3–7% of the build cost |
| Structural engineer | £1,500 – £2,500 |
| Planning application (householder, England) | £528 + £85 Planning Portal fee |
| Building Control fees | £600 – £1,200 |
| Party wall agreement (per neighbour) | £1,500 – £3,000 |
| Building insurance (for a £100,000 extension) | £1,500 – £3,000 |
| VAT | usually 20% on building work |
| Contingency | at least 10%, plus around 5% for price rises |
Do You Need Planning Permission for an Extension?
Many single storey rear extensions in England fall under permitted development, so a full planning application is not always needed. As a general guide, a single storey rear extension can usually extend up to 3m from the back of an attached house, or 4m for a detached house, and can be larger through the prior approval process. Two storey extensions, side extensions over a certain size, and homes in conservation areas or listed buildings usually need planning permission. Always check with your local planning authority before you start.
Almost every extension needs Building Regulations approval, covering foundations, structure, insulation, drainage, ventilation and fire safety. You will need planning drawings where planning is required, and building regulations drawings for Building Control.
How Long Does a House Extension Take?
Most house extensions take around 3 to 6 months on site, depending on size and complexity. A simple single storey rear extension may take around 3 months, while a double storey or complex glazed extension takes longer. Allow extra time before work starts for design, drawings and approvals.
House Extension Cost Breakdown: Where the Money Goes
A typical extension budget is split across:
- Groundworks: excavation, foundations, drainage and the ground floor slab
- Walls: brickwork, blockwork, cavity insulation and lintels
- Structural steel: beams and padstones to open up the existing house
- Roof: structure, coverings, insulation and rooflights or lanterns
- Windows and doors, including bifold or sliding doors
- First fix and second fix electrics, plumbing and heating
- Plastering, flooring, decoration and the kitchen, if included
- Preliminaries: scaffolding, skips, welfare and site management
A line-by-line house extension estimate shows each of these separately, so you can check quotes and see exactly where the money goes.
How to Keep House Extension Costs Down
- Keep the shape simple: a rectangle is cheaper to build than a complex footprint.
- Consider a double storey extension if you need lots of space, as it is cheaper per m².
- Limit expensive glazing, or choose standard sizes for doors and windows.
- Get full drawings before asking for quotes, so every builder prices the same scope.
- Ask for itemised quotes rather than a single lump sum.
- Keep a contingency and agree how changes will be priced before work starts.
How to Get an Accurate House Extension Price
Average prices are a useful starting point, but the only way to know what your extension will cost is to price it from drawings. At Nextron Estimating, we prepare detailed house extension estimates for UK builders and contractors, with quantities, materials and labour priced line by line.
- House extension estimating: an itemised price from your drawings
- House extension design drawings: from concept to planning and technical drawings
- Loft conversion cost guide: compare the cost of converting your loft
Frequently Asked Questions
How much does a house extension cost in the UK?
Most UK house extensions cost £1,800–£3,300 per m² in 2026, before VAT. A 20m² single storey extension typically costs £40,000–£56,000.
How much does a single storey extension cost?
Around £40,000–£56,000 for 20m², and £60,000–£84,000 for 30m², before VAT. Higher-specification finishes cost more.
How much does a double storey extension cost?
Around £2,000–£3,200 per m², or £108,000–£180,000 for a 60m² extension, before VAT. It is usually cheaper per m² than a single storey extension.
How much does a kitchen extension cost?
A 30m² kitchen extension typically costs around £66,000–£99,000 plus VAT. The kitchen itself often adds £15,000–£35,000.
Is it cheaper to build a double storey extension?
Per square metre, yes. The foundations and roof are shared across two floors, so you get much more space for a smaller increase in cost.
How long does an extension take to build?
Usually 3–6 months on site, plus time beforehand for design and approvals.
Do I need planning permission for a house extension?
Not always. Many single storey rear extensions fall under permitted development, but double storey and larger side extensions usually need planning permission. Building Regulations approval is almost always required.
Sources
Get a House Extension Estimate
Send us your drawings and we will prepare a clear, itemised house extension estimate. If you want to see our format first, ask for a free sample estimate.
Call us on +44 7576 532125, or email info@nextronestimating.co.uk.
