Extension costs: how much should you expect to pay in 2024?

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 Royal Institution of Chartered Surveyors (RICS) to access pricing information from the Building Cost Information Service (BCIS) database. 

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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.

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Single-storey extension with one window

Double-storey extension with two windows

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What to consider before you start

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.

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.

Which? Trusted Traders has a great library of vetted and reliable traders in your local area. 

To discover the key questions you should be asking your builder, and what you can expect from an architect, read this advice from a builder and architect