How much does it cost to add an extension to my house?

As a leading Design and Build company, we are often asked by our clients, “How much will my house extension cost?” We’re firm believers in designing with our client’s budget being at the forefront of our minds. In fact there are three main advantages of choosing a design and build company – easy, speedy and cost certainty. 

In short, most extension projects cost around £1750-£2750/m² of new internal space. So a 30m² kitchen extension could be estimated at somewhere between £52,500-£82,500, plus, VAT at 20%. 

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If that sounds suitably general — that’s because it is. And that’s all you can expect when you have no detail or idea of what the extension looks like, or what materials you’re going to use, or how it is going to be built (and who’s going to do it). The £1750-£2750 price, however, is a very good range of pricing based on averages.

There are multiple factors that can affect your project’s costs, including:

  • How many storeys you’re going to build. 
  • The size and shape of the extension.
  • The quality of the build: standard, good, excellent.
  • The build route you’ll take, how involved in the project you’ll be.
  • The amount of glazing you’d like.
  • Whether the extension contains a kitchen or bathroom. 

What other costs are there?

Here are some example costs you might need to factor in:

  • Architectural fees (check out our instant fees)
  • Structural engineer (approx. £500-£1,500). 
  • Surveys (approx. £700-£1,800).
  • Planning permission (if needed — currently £258 for a householder fee,  in England for a two-storey extension.
  • Lawful Development Certificate (currently £103).
  • Building Regulations applications (approx. £800-£1,800).
  • Fitting out the extension (costs will vary depending on how you plan on using the new space).
  • Party Wall Agreement (if needed — £1,000-£2,000 per affected neighbour for a straightforward agreement outside London).
  • Insurances.

The overall quality of the extension will impact the estimated cost of the project, the costs quoted below are for an insulated cavity wall extension with 100mm blockwork and faced with brick. 

A standard quality extension represents most speculative developers concrete interlocking roof tiles, studwork partitions, contract kitchen and radiator central heating. 

Good quality extensions also use better quality facing bricks clay roof tiles, high-end softwood joinery, blockwork partition walls, a top-of-the-range kitchen and underfloor heating. 

An excellent quality extension sees top-of-the-line facing bricks, play clay tile roofs, hardwood joinery, a bespoke kitchen, high quality sanitary wear and underfloor heating. 
Standard quality: £1750 to £2000/m²

Good quality: £2000 to £2500/m²

Excellent quality: £2500 to £2750/m² or more.

For an excellent finish you can typically expect to pay 40% more than a standard finish.

NOTE: The indicative figures used in this calculator were last updated in 2024 and do not include the cost of the site, professional fees, external works or any VAT.

Do you pay VAT on extensions?

Building work to extend an existing house is usually subject to VAT at the standard rate, currently 20%. If you’re building an entirely new dwelling then you will be VAT exempt. 

The construction work for some extension projects are eligible for the reduced rate of VAT, currently 5%. These include extensions as part of:

  • the conversion of an existing dwelling that changes the number of units.
  • work to a building that has been unoccupied for at least two years.

To benefit from VAT relief from the above, you must use a VAT registered builder — you can’t reclaim the VAT yourself.

Will an extension add value to my home?

Before getting too wrapped up in the specifics of extension costs, take time to consider whether an extension is financially viable and right for your property. 

Is this your forever home? If so, an extension is a no-brainer compared to the hassle of moving house. In a financial sense, opting to add even small house extension ideas means you’ll save on moving costs, solicitors’ fees and stamp duty — costs which aren’t recouped by adding value in the same way as your extension. However, consider that increasing the size of your home may move you into a higher council tax bracket.

If you are planning on selling up, whether soon or in the future, you’ll need to ensure that you add more value to your home than the extension costs. Speak to local estate agents and research ceiling prices in the area before you decide to go ahead. 


Adding an extension to your house is a significant investment that requires careful planning and budgeting. By understanding the various costs involved and budgeting accordingly, you can ensure that your extension project stays on track and delivers the desired results. Whether you’re expanding your kitchen, adding a new bedroom, or creating a home office, investing in an extension can enhance your living space and improve the value of your home for years to come.

To find out more please book a free consultation. We would be happy to help!



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