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Extension costs: how much should you expect to pay in 2025?
There's no doubt that extensions cost a lot of money, but a lot of the time they're well worth the investment.
It's always a sensible idea to get two or three quotes before you commit.
To give you an idea of how much an extension costs, we've worked with the Building Cost Information Service (BCIS) to bring you the latest pricing information.
Whether you opt for a loft conversion, porch or garage conversion, there are key points to consider before you start. Here's how to plan an extension
How much does an extension cost?
If you're a Which? member, log in to unlock pricing data from BCIS for various sizes of extensions.
If you're not yet a member, join Which? to get instant access.
Single-storey extension with one window
| Floor size (metres) | Flat roof | Pitched roof |
|---|---|---|
| 3 x 3 | Sign up to reveal Get instant access to this and all our scores and recommendations Unlock tableFull Access first month £5, then £11.99 per month, cancel at any time. Already a member? Log in | |
| 3 x 5 | ||
| 4 x 4 | ||
| 4 x 6 | ||
Sign up to reveal
Get instant access to this and all our scores and recommendations
Unlock tableFull Access first month £5, then £11.99 per month, cancel at any time.
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Double-storey extension with two windows
| Floor size (metres) | Flat roof | Pitched roof |
|---|---|---|
| 3 x 3 | Sign up to reveal Get instant access to this and all our scores and recommendations Unlock tableFull Access first month £5, then £11.99 per month, cancel at any time. Already a member? Log in | |
| 3 x5 | ||
| 4 x 4 | ||
| 4 x 6 | ||
Sign up to reveal
Get instant access to this and all our scores and recommendations
Unlock tableFull Access first month £5, then £11.99 per month, cancel at any time.
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What to consider before starting your home extension project
Before you take the plunge, consider these points:
- When setting a budget for your job, start by making a list of everything you would like included.
- Bear in mind the cost of a two-storey extension isn't simply double the cost of a single-storey structure, as much of the cost is in the foundations. Per square metre, it will work out cheaper.
- Compare your quotes item by item when pricing services and materials.
- Always check that VAT is included.
- Set aside around 10-15% of your budget for contingencies (unexpected additional costs).
- Remember that most builders take payments in stages.
- Fit-out costs for extras such as a kitchen, utility room, sliding doors, underfloor heating etc, will be additional.
- A new boiler may be needed to meet the demand of water-fed underfloor heating; the Energy Saving Trust estimates the cost of a gas boiler replacement at around £2,300.
- Having a building contractor or architect to manage your project will cost you more, but remove a lot of the stress. Budget for architect design fees up to 10% of the construction cost.
- Planning drawings, construction drawings, a measured survey of the existing house and a structural engineer to size roof joists and foundations will all cost extra.
Get advice from a builder and an architect on where to start with a home extension
Looking for a good builder?
The best place to start is to try and find someone who has been through the process before and get their recommendations for good local architects, structural engineers and builders.
Read our guide to finding a reputable trader, or use Which? Trusted Traders to search for vetted, reliable traders in your local area.
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