How Much Does a House Extension Cost? UK 2026 Prices
A house extension is one of the most popular ways to add space and value. But costs vary hugely depending on size, type and specification. Here is a comprehensive guide.
Extension Costs by Type
| Extension Type | Cost per m2 | 20m2 Extension |
|---|---|---|
| Single-storey rear | £1,500–£2,500 | £30,000–£50,000 |
| Single-storey side return | £1,800–£3,000 | £36,000–£60,000 |
| Double-storey | £1,200–£2,000 | £48,000–£80,000 |
| Wrap-around | £1,500–£2,500 | £45,000–£75,000 |
| Over-garage | £1,200–£1,800 | £24,000–£36,000 |
| Conservatory | £800–£1,500 | £16,000–£30,000 |
Note: Double-storey extensions cost more than single-storey but give you twice the space, making them better value per m2.
Hidden Costs to Budget For
Architect/designer: £2,000–£5,000 for plans. Planning application: check the Planning Portal fee calculator — England fees are index-linked and change each April. Building regulations: £500–£1,000. Structural engineer: £500–£1,500. Party wall surveyor: £800–£1,500 per neighbour. Temporary kitchen/bathroom: If extending your kitchen, budget for a temporary setup. Landscaping: Making good the garden after construction.
Planning Permission
Many single-storey rear extensions fall under Permitted Development rights (up to 3m for attached houses, 4m for detached). Larger extensions, side extensions, and two-storey extensions usually require full planning permission, and the work itself will need an extension builder who can evidence building control sign-off. Always check with your local planning authority before starting work.
Where the rules bite is in the detail — the eaves height, the boundary distance, and whether an Article 4 direction has removed the right at your address. A guide to planning permission for a rear extension covers the current limits, and if the scheme needs a full application it will be decided on the drawings rather than on the description.
Getting the Drawings Right First
The £2,000–£5,000 designer line in the hidden-costs list is the one that decides the accuracy of every other number on this page. Builders quote against a drawing set; without one they quote against their own assumptions, and the difference only surfaces as variations once work has started.
A full package covers the measured survey, existing and proposed plans and elevations, the planning application set, and the building-regulations drawings your contractor and building control both work to. A chartered practice such as Architectural Drawings handles design through to approval, while Drawings Direct offers the same rear-extension drawing scope on a fixed fee where you already know what you want built.
Frequently Asked Questions
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