The short answer
House extension costs in the UK typically range from £20,000 for a modest single-storey addition to £100,000+ for a large double-storey project, with London and the South East costing 20–40% more than the Midlands or North. Cost per m² varies between £1,500 and £3,500+ depending on spec and location. See single-storey extension cost and double-storey extension cost for the breakdowns.
Extension costs are one of the most searched questions in home improvement, and also one of the most misleading because the figures quoted online cover an enormous range of projects. A 15 m² kitchen extension built to a standard specification in the Midlands is a fundamentally different proposition from a 60 m² wrap-around in London with underfloor heating, glazed roof and high-end kitchen. This guide gives you the honest ranges, explains what drives the price and shows you how to get quotes that are actually comparable.
Extension costs at a glance (2026, UK)
- Single-storey (15–30 m²) £20,000–£45,000 typical
- Double-storey (25–50 m² added) £40,000–£90,000 typical
- Wrap-around (30–60 m²) £50,000–£120,000 typical
- Cost per m² £1,500–£3,500+ depending on spec
- London premium 20–40% higher than Midlands/North
- Additional costs Architect, planning, structural engineer: £3,000–£10,000+
What drives the cost of a house extension?
The two biggest variables are size and specification. Beyond those, the main cost factors are:
- Location: London and the South East typically cost 20–40% more than the Midlands, North or Wales due to labour rates and access costs.
- Ground conditions: if the soil is made ground, clay subject to movement, or close to trees, the foundation design becomes more complex and costly. A structural engineer’s input is essential before the foundation type is determined.
- Structural complexity: removing load-bearing walls, installing RSJ beams or large structural steel frames adds significant cost over a simple extension onto a non-structural wall.
- Glazing: roof glazing, large sliding or bifold doors and structural glazing are beautiful but expensive. A glazed flat roof can cost £5,000–£15,000 on its own.
- Specification: the finish quality of kitchen, flooring, underfloor heating and joinery varies enormously and accounts for a large share of the final cost.
- Existing services: rerouting drainage, gas or electricity adds cost and time.
| Extension type | Typical size | Typical cost (Midlands/North) | Typical cost (London/SE) |
|---|---|---|---|
| Single-storey rear | 15–30 m² | £20,000–£40,000 | £28,000–£55,000 |
| Single-storey side | 10–25 m² | £18,000–£35,000 | £25,000–£48,000 |
| Double-storey rear | 25–50 m² added | £40,000–£75,000 | £55,000–£100,000 |
| Wrap-around (L-shape) | 30–60 m² | £50,000–£90,000 | £70,000–£120,000+ |
Professional fees: the costs beyond the build
The builder’s contract price is not the total cost. Allow for:
- Architect or designer: £2,000–£6,000+ for a domestic extension, depending on complexity and the level of service (concept only vs full contract administration).
- Structural engineer: £500–£2,000 for foundation design and structural calculations.
- Planning application fee: £258 in England (2026 rates — confirm with your LPA).
- Building regulations fee: varies by local authority and project value; typically £400–£1,200.
- Party wall surveyor: £700–£2,000 per side if a party wall agreement is needed.
What does cost per m² actually mean?
Cost per m² is a useful shorthand for comparing quotes but must be used carefully. The figure varies enormously with specification: a standard finish might cost £1,500–£1,800/m²; a high-specification extension with roof glazing, underfloor heating and quality fittings can reach £2,500–£3,500/m² or more. Small extensions also cost more per m² than large ones, because fixed costs (scaffolding, site setup, temporary works) are spread over fewer square metres. Use cost per m² to sense-check quotes, not to generate your own budget.
How to get accurate quotes
Accurate quotes require a design — ideally drawn by your architect — so that all builders are pricing exactly the same scope of work. Quotes based on a verbal description or a sketch will vary wildly and are not comparable. Once you have drawings, get at least three quotes from builders with demonstrable experience of extensions and references you can follow up. Ask for a fixed-price contract (or a schedule of rates with quantities), a payment schedule tied to stage completion, and a written programme. See how to choose a builder for an extension for the full checklist.
VAT on extension costs
New construction work, including most house extensions, is zero-rated for VAT, meaning the builder should not charge VAT on the supply and fix of materials and labour. There are exceptions and grey areas (conversion work, certain listed building scenarios), so confirm the VAT position in writing with your builder. If a quote from a VAT-registered builder is significantly lower than others, check whether VAT has been omitted rather than whether the price is genuinely lower.
All costs on this page are typical illustrations drawn from trade guidance and not quotes for your specific project. Actual costs depend on your design, ground conditions, specification and the contractors available in your area. Always obtain written quotes before committing.
Get written extension quotes
To find out what your specific extension would cost, you need written quotes from builders who have seen your drawings. We can connect you with experienced contractors — no obligation.
Frequently asked questions
How much does a single-storey extension cost in the UK?
A typical single-storey rear extension of 15–30 m² costs £20,000–£45,000 in the Midlands or North, and £28,000–£55,000 in London and the South East, depending on specification and ground conditions.
Is a double-storey extension cheaper per m²?
Usually yes. The foundations and ground-floor structure are shared across two levels, so the cost per m² of added space is typically lower for a double-storey extension than a single-storey one of equivalent footprint.
Do I pay VAT on a house extension?
Most new extension construction work is zero-rated for VAT. However, there are exceptions, and you should confirm the VAT position in writing with your builder before signing any contract.
What is included in the cost of a house extension?
A typical build contract includes groundwork, foundations, brickwork, roofing, glazing, plastering, electrical first fix and second fix, plumbing and drainage. It usually excludes kitchen, flooring, decoration and furniture, unless specified. Always confirm exactly what is included before signing.
Sources & further reading
- RICS — Guidance on residential construction costs and building surveys
- LABC — Building regulations fees and compliance for home extensions
- Planning Portal — Planning application fees for England (2026)
- GOV.UK — VAT: construction and building services guidance (Notice 708)
This is general information about house extensions in England and is not planning, structural, legal or financial advice. Costs, timescales and outcomes vary with your design, ground conditions, specification and local authority. Always obtain written quotes and verify planning and building regulations requirements with your local planning authority before committing to any works.