A house extension in London typically costs £1,800–£3,000 per m² for a standard to mid-range specification. A typical 20m² single-storey rear extension costs £36,000–£60,000 before VAT, professional fees, and finishes. Double-storey extensions cost £2,500–£3,500 per m². London costs run approximately 15–25% above the national average.
House Extension Costs by Type (London, 2025)
The type of extension you build is the first major cost variable. Rear extensions are the most common and most cost-effective per m². Side and wrap-around extensions tend to cost more due to structural complexity and, often, the need for planning permission.
| Extension Type | Typical Size | London Cost Range | National Average |
|---|---|---|---|
| Single-storey rear extension | 15–30m² | £27,000–£90,000 | £18,000–£60,000 |
| Double-storey rear extension | 30–60m² | £60,000–£180,000 | £40,000–£120,000 |
| Single-storey side extension | 10–20m² | £18,000–£60,000 | £12,000–£40,000 |
| Side-return extension | 8–15m² | £20,000–£50,000 | £14,000–£35,000 |
| Wrap-around extension | 25–50m² | £55,000–£150,000 | £35,000–£100,000 |
| Two-storey side extension | 20–40m² | £50,000–£140,000 | £35,000–£90,000 |
* All costs exclude VAT (20%), professional fees, and finishes. Based on standard to mid-range specification.
House Extension Costs by Size (London, 2025)
The table below shows indicative costs for single-storey rear extensions in London at three specification levels. Basic covers a standard brick-and-block build with budget fixtures. Mid-range includes better-quality materials and a more considered finish. Premium covers structural glazing, underfloor heating, and high-specification fittings.
| Floor Area | Basic (£1,800/m²) | Mid-Range (£2,200/m²) | Premium (£3,000/m²) |
|---|---|---|---|
| 10m² | £18,000–£22,000 | £22,000–£30,000 | £30,000–£45,000 |
| 15m² | £27,000–£33,000 | £33,000–£45,000 | £45,000–£67,500 |
| 20m² | £36,000–£44,000 | £44,000–£60,000 | £60,000–£90,000 |
| 25m² | £45,000–£55,000 | £55,000–£75,000 | £75,000–£112,500 |
| 30m² | £54,000–£66,000 | £66,000–£90,000 | £90,000–£135,000 |
| 40m² | £72,000–£88,000 | £88,000–£120,000 | £120,000–£180,000 |
* Costs exclude VAT, professional fees, and external works. London pricing.
What Affects the Cost of a House Extension?
The per-m² cost is a useful starting point, but the final price of your extension depends on a combination of factors. Understanding these helps you make informed decisions about where to invest and where to save.
The floor area is the single biggest cost driver. London builders price extensions at £1,800–£3,000 per m² for standard builds, rising to £3,500+ for premium specifications.
A basic extension uses standard brick, concrete block, and budget-grade fixtures. A premium extension uses structural glazing, underfloor heating, bespoke joinery, and high-end kitchen or bathroom fittings.
Removing load-bearing walls, installing steel beams, underpinning foundations, or working on a sloped site all add cost. A structural engineer's report is required for any structural alterations.
Labour costs vary across London. Central and West London builders typically charge 15–25% more than East London and Hertfordshire builders for equivalent work.
Architect fees typically run 5–10% of build cost. Planning applications cost £258 for a householder application. Building Regulations fees are set by your local authority.
If your extension is within 3m of a neighbour's property, the Party Wall Act 1996 applies. Surveyor fees for a straightforward agreement typically run £700–£1,500 per neighbour.
Cost Breakdown: Labour, Materials & Professional Fees
For a typical 20m² single-storey rear extension in East London at mid-range specification (total build cost approximately £44,000–£60,000 before VAT), the cost breakdown typically looks like this:
Groundworks, bricklaying, carpentry, plastering, electrics, plumbing, decoration
Foundations, blockwork, roof structure, windows, doors, insulation, finishes
Design, planning drawings, Building Regulations drawings, contract administration
Structural calculations, steel beam specification, foundation design
Planning application (if required), Building Regulations full plans application
Site management, insurance, waste disposal, plant hire
How to Get an Accurate Quote for Your Extension
Online cost calculators and per-m² guides give you a useful starting range, but they cannot replace a proper site survey and itemised quotation. The only way to get an accurate price for your specific extension is to have a builder visit your property.
When requesting quotes, always ask for an itemised breakdown — not a single lump sum. This allows you to compare quotes meaningfully and understand exactly what is and is not included. Ask specifically whether the quote includes VAT, professional fees, Building Regulations, and external works.
Get at least three quotes. The cheapest quote is not always the best value — look for a builder who can demonstrate relevant experience, provide references, and offer a fixed-price contract rather than a day-rate arrangement.
TCM Building & Maintenance provides free site surveys and detailed, itemised quotations for house extensions across London and Hertfordshire. No obligation. No vague estimates.