How much does a house extension cost in 2026?

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.

How much does a house extension cost in the UK? 

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

Looking for some inspiration? Take a look at our home extension and conversion ideas

How to reduce extension costs

Bear in mind the cost of a two-storey extension isn't simply double the cost of a single-storey structure, because much of the cost is in the foundations. A double-storey extension will actually work out cheaper per square metre, so think about how much space you need now, and in the long term, before taking the plunge.

Design and planning

• Keep your design simple: Rectangular or square footprints with simple pitched or flat roofs are significantly cheaper than curved walls or vaulted ceilings.
• Avoid bespoke glazing: Design your openings around standard-sized windows, doors, and roof lights. Custom-made glass can add thousands to your final bill.
• Pre-empt hidden fees: Planning drawings, construction drawings, a measured survey of the house and a structural engineer to size roof joists and foundations will all cost extra. Consider any regulation or structural issues before starting, such as the unknown presence of asbestos, or the need for a bat survey – both can be surprisingly expensive.

Budgeting and quotes

• Set your budget: Use our pricing data above as a guide to what you can expect to spend, establish your maximum funds, and budget for architect design fees up to 10% of the construction cost. Bear in mind that fit-out costs for extras such as a kitchen, utility room, sliding doors, underfloor heating etc, will be additional. Set aside around 10-15% of your budget for contingencies (unexpected additional costs). 
• Get at least three quotes: Get detailed, itemised quotes and compare them line by line when pricing services and materials. Always check that VAT is included. 
• Source your own finishes: Although builders often get trade discounts on construction materials like timber and bricks, you can often save money by sourcing your own finishes, like tiles, flooring and kitchen units.
• Spend now to save later: Don’t cut corners on efficiency. Investing in good insulation, double glazing and upgrading to a more energy-efficient boiler may seem pricey, but it will be a lot more expensive – and disruptive – to retrofit later. Plus, it will lower your long-term utility bills. Use our home energy planning tool to create a more energy-efficient home.
• Consider cash flow: Most builders take payments in stages, so ensure you set out a clear payment schedule - and don’t forget that contingency fund.

Labour and project management

• Act as project manager: Hiring a main contractor to oversee everything is certainly less stressful and can make the whole process much more seamless. But managing individual tradespeople yourself can save you a lot of money.
• DIY the finishing touches: You don't need to be a qualified builder to handle the decorating. If you’re confident in your DIY skills, consider taking on the painting, tiling and flooring yourself once the structural work is finished.

Does an extension add value to my house?

A huge variety of factors determine the value of a property, such as location, condition and style. However, generally speaking, an extension will add value to your house, particularly if it increases the number of bedrooms. 

Research conducted by Nationwide in October 2025 found that the following types of extension can add value to your property: 

• An extension or loft conversion that includes a large double bedroom and a bathroom can increase the value of a three-bedroom, one-bathroom house by as much as 24%.
• Expanding the floor area of your home by 10% could add up to 5% value.
• Adding an extra bathroom on its own could add up to 4%.

This table shows how the amount of value added varies by property type and size:

Looking for a good builder?

It's worth asking friends and neighbours to recommend local architects, structural engineers and builders, but we've done the hard work for you.

Use Which? Trusted Traders to search for vetted, reliable traders in your local area:

For further advice on finding the right person for the job, read our guide to finding a reputable trader