how much does a house extension cost
Extensions are big-ticket jobs. They are also where contractors lose the most money. A kitchen extension that looked straightforward on paper turns into a nightmare when you hit clay, the steel takes eight weeks, and the building inspector wants a full drainage recalculation. Get your pricing wrong and you are working for free by week ten.
This guide covers real extension costs in 2026. UK prices are the baseline. If you are quoting in the US or Australia, expect labour rates 20-40% higher and material costs that vary by region — but the risk factors are the same everywhere.
single-storey vs two-storey
A single-storey rear extension (roughly 4m x 5m) typically lands between £30,000 - £60,000 in the UK. Two-storey extensions are not double the cost — more like 50-70% more — because you share the foundations, roof structure, and most of the groundwork. Expect £50,000 - £95,000 for a two-storey of similar footprint.
The per-square-metre rate for a single-storey sits around £1,500 - £2,500. Two-storey drops to £1,200 - £2,000 per sqm because you spread the expensive stuff (foundations, roof) across more floor area.
In the US, a similar single-storey addition runs $40,000 - $80,000 depending on state. In Australia, $50,000 - $120,000 AUD. The maths changes but the risk profile does not.
where extension costs blow up
Extensions have more risk than most contractors price for. Here is where the money disappears:
- Foundations. You will not know the full picture until you dig. Clay soil, tree roots, old drains in the way — any of these can double your groundwork cost. If the site investigation is thin, price foundations as High uncertainty.
- Planning and building regs. Permitted development is not guaranteed. Party wall agreements drag on. Building control might want engineered details you did not budget for. Allow time and money for delays.
- Structural steel. Lead times in 2026 are still unpredictable. A standard RSJ might take two weeks or eight. If the steel is late, everything else stacks up behind it. Price the wait into your programme.
- Weather. Groundwork and brickwork are weather-dependent. A wet autumn can add weeks to a programme. If you are starting in October, build that into your quote.
worked example: single-storey rear extension (4m x 5m)
| Item | Cost Range | Risk Level | Notes |
|---|---|---|---|
| Foundations & groundwork | £4,000 - £10,000 | High | Soil type is everything. Budget for the worst. |
| Structural steel (RSJs) | £1,500 - £3,500 | High | Lead times are volatile. Order early. |
| Brickwork & blockwork | £4,000 - £7,000 | Medium | Matching existing brick adds cost |
| Roof structure & covering | £3,000 - £5,500 | Low - Medium | Flat roof = simpler. Pitched = more labour. |
| Windows & bifold doors | £3,000 - £8,000 | Medium | Lead times on aluminium bifolds: 4-8 weeks |
| Electrics (first fix + second fix) | £1,500 - £3,000 | Low | Standard domestic. Consumer unit upgrade adds £500. |
| Plumbing & heating | £1,200 - £2,500 | Low - Medium | Extending the existing system. New boiler = separate quote. |
| Plastering & drylining | £1,500 - £2,500 | Low | New build walls = predictable |
| Flooring & decoration | £1,500 - £3,500 | Medium | Customer spec drives this line completely |
| Drainage & building regs | £1,500 - £3,000 | High | Sewer connections, inspections, sign-off |
Total range for a single-storey rear extension: £22,700 - £48,500. The low end assumes good ground, standard finishes, and no planning complications. The high end is where reality usually lives.
quoting extensions without losing money
A flat 10% contingency does not cut it on extensions. Your foundations might be fine or they might need piling. Your steel might arrive in two weeks or two months. Rate each line item individually. The plastering is Low risk — you know exactly what it costs. The groundwork is High risk — it depends on what is in the ground. Feed honest ratings into a proper simulation and your quote covers the real spread of outcomes, not a guess.