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Extension cost by floor area
The table multiplies each floor area by the current £1,800–£3,000 per m² benchmark. It is useful for early budgeting before the project is detailed enough for itemised quotes.
Popular extension dimensions
Dimensions are simply another way of expressing floor area. A 4m × 5m footprint is 20m², while a 5m × 5m footprint is 25m².
9m² · Compact kitchen widening, office or utility space£16,200–£27,000
12m² · Small rear or side-return extension£21,600–£36,000
15m² · Kitchen-diner or family space£27,000–£45,000
16m² · Popular square kitchen-diner footprint£28,800–£48,000
20m² · Typical larger kitchen-diner footprint£36,000–£60,000
25m² · Large open-plan family room£45,000–£75,000
What changes the cost per m²?
- Region: London, the South East and difficult urban access can lift labour and logistics costs.
- Structure: large knock-throughs, steel beams, unusual foundations and drainage diversions add cost independently of floor area.
- Glazing: large sliding doors, aluminium systems, roof lanterns and multiple rooflights can move the project well above a basic shell.
- Services: relocating heating, electrics, drainage or gas increases labour and coordination.
- Fit-out: kitchens, bathrooms, flooring and bespoke joinery can be a major share of the finished budget.
- Access: restricted side access, parking controls, scaffolding and manual waste removal can increase preliminaries.
How to use the size table
Start with the floor-area range, then treat it as the core build allowance rather than a final budget. Add any known kitchen or bathroom package, check whether VAT and professional fees are included, then keep a contingency for unknown conditions and design changes.
For example, a 20m² extension gives a straight-line core range of £36,000–£60,000. A contractor quote should then make clear whether structural steel, glazing, plumbing, electrics, flooring, decoration, waste, VAT and professional fees sit inside or outside that figure.
Regional extension pricing
UK Home Costs uses regional modelling to adjust the same national benchmark rather than claiming a separate observed average quote for every city. Open a regional hub for context or use the postcode-aware calculator for a project estimate.
Source and methodology
House Extension: Cost Breakdown 2026
Published: June 2026
Used for: national extension cost-per-m² benchmark and common project cost context.
The page deliberately keeps the source benchmark separate from UK Home Costs regional and specification modelling. See methodology for how adjustments are handled.
Frequently asked questions
How much does a 20m² extension cost in the UK in 2026?
Using the current £1,800–£3,000 per m² national benchmark gives a straight-line planning range of about £36,000–£60,000 before VAT. Structural work, glazing, kitchens, bathrooms, access and location can move a real quote outside that range.
How much does a 30m² extension cost?
A 30m² extension works out at about £54,000–£90,000 before VAT using the same £1,800–£3,000 per m² planning benchmark.
Why can a small extension cost more per square metre?
Foundations, design work, structural openings, site setup, waste removal and other fixed costs do not shrink in proportion to floor area. Very small extensions can therefore have a higher effective cost per square metre.
Does the extension cost per m² include a kitchen or bathroom?
Do not assume so. Kitchen and bathroom fit-outs, premium glazing, complex steelwork, professional fees and VAT should be checked explicitly in every contractor quote.
Are London extension costs higher?
Often yes. Labour, access, parking, logistics and contractor overheads can raise costs in London and parts of the South East. Use the postcode-aware calculator for a regional planning adjustment.