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How Much Does an Extension Cost?

how much does an extension cost
(Image credit: Simon Burt)

How much does an extension cost? It's often the first question asked by those thinking about adding extra space to their home and it's key to understanding what you'll be able to build. Budget is often the biggest driver as to whether a project will ever leave the drawing board.  

Whether you've considering a single or two-storey build, there are many factors involved in determining the costs:

  • How many storeys you're going to build 
  • The size 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
  • And don't forget kitting out the room it'll be eg the cost of a kitchen on top of the extension costs.

To help you better understand what your budget might get you in the way of building an extension, we've put together five top tips to help you calculate costs.

Use this guide alongside our free extension cost calculator to estimate how much your project is likely to cost.

(MORE: Get a tailored quote for your new extension)

1. How Much Does a Single or Two-Storey Extension Cost?

The cost of a straightforward box-shaped extension should allow for around £1,000–£2,000/m² but prices will depend on where you are in the UK. 

How you finish the extension, eg the standard of spec you choose will heavily influence the build costs, too.

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

Expect to pay roughly…

  • Basic quality: £1,320 to £1620/m²
  • Good quality: £1,700 to £2,000/m²
  • Excellent quality: £1,800 to £2,500/m² or more.

Bear in mind economies of scale on larger extensions. A two-storey extension will not cost much more per m2 because, aside from the extra interior fixtures and finishes, you are only adding walls and floor joists — a roof and foundations are required whether your extension is single or two storey.

The cost of a side extension, for example, will depend largely on the size of the structure, the location of the build and the quality of the materials and finishes. “A basic range of costs might be £1,500 - £2,000 per m2, with smaller extensions generally having a higher per m2 cost than larger ones,” says Nicola Chambers director at Pardon Chambers Architects.

Don't over spend. Remember, balance the amount you are willing to spend on your extension with the estimated value it will add to your home.

Editor's Note: Homebuilding.co.uk partners with the UK's best extension specialists to match your requirements with their services. Simply answer a few questions on what you need from your extension and we’ll put you in touch with a suitable partner.

2. What Other Extension Costs Should I Factor in?

From the size of your build to the soil type you're going to build on - there are many factors to think about when costing up your extension. Here's an example:

  • soil type (which affects the foundations)
  • what the extension will be used for (and how much it will cost to fit it out)
  • how much glazing you plan to specify
  • size (it gets slightly cheaper per unit area as the size increases)
  • location (the most expensive locations are around 30% more than the cheapest locations when it comes to labour).

3. Who Will Manage my Extension Project?

How much does an extension cost

The homeowner of this London flat learned how to tile, lay flooring and decorate to a high standard to achieve the look she wanted in her extension and renovation project (Image credit: Effie Dracou)

Known in the trade as the build route this determines who you will choose to manage your project. The route you opt for will of course affect the cost of your extension project.

If you're a keen and competent DIYer who will take on most of the work yourself then this could mean a 40% variation between this - probably the cheapest route - and taking on a main contractor who also takes on the role of PM.

There are four main routes to choose:

  • DIY: building on a largely DIY basis, substituting around 30% of the labour costs with DIY and employing help with the rest of the building work. Materials purchased directly
  • Self managed/subcontractor: building using tradespeople hired directly. Minimal DIY involvement. Most materials are purchased directly
  • Hiring a main contractor and subcontractors: building using a main contractor or package supplier to complete the extension to a weathertight stage, with the remaining work being undertaken by subcontractors with most materials purchased by self builder direct from suppliers
  • Hiring a main contractor: building using a main contractor. Building in this way requires the least involvement from the self-builder.

4. What's the Difference Between a Quote and an Estimate?

It sounds obvious, but not being clear about quotes and estimates can make a real difference to the accuracy of your pricing and how much your extension will cost.

An estimate is normally a contractor’s guess as to what your extension will cost. Whether given verbally, or in writing, is not legally binding and the final bill may exceed it.

A quotation is a definite price. The written quotes should itemise the work to be done, provide a breakdown of costs and a total, and state whether VAT is included.

Tips for Comparing Quotes

  • When you receive the bids, check whether there are any caveats that might involve extra expense
  • Compare provisional sums for work such as foundations to make sure you are comparing like with like

5. Do I Have to Add VAT to the Cost of my Extension?

Most extensions will be subject to VAT on labour and materials at the standard rate of 20%, especially if you use a contractor to undertake the work.

If you use local tradespeople who are not VAT registered you can save the 20% VAT on their labour, but you will still have to pay VAT on materials at the standard rate.

Some extension projects are eligible for VAT relief, such as:

  • 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.