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How Much Does an Extension Cost?

How much does an extension like this single-storey cost
A hidden sliding glass pocket door on this single-storey extension allows the inside to connect to the garden seamlessly (Image credit: IQ Glass)

Wondering how much does an extension cost? Costing up your project is one of those make or break moments. Will your budget stretch to the costs you'll likely pay? 

After all, there are so many factors to think about that it's vital your costs are calculated before you even consider breaking ground. 

Here we explain the factors that will affect the cost of building an extension and give you an idea of cost per m2 so you can estimate your potential costs.

Use this guide in conjunction with our free extension cost calculator to accurately estimate the likely cost of your project.

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

How much an extension will cost will depend on factors like:

  • 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 get an idea of how much your extension will cost, we’ve pulled together five top tips that will help you set a budget before you begin.

Here’s what to consider.

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

Depending where you are in the UK, the cost of a straightforward extension should allow for around £1,000–£2,000/m². The standard of specification you choose will have an enormous influence on the build cost.

On average, an excellent finish is 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.

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. Factor in Other Extension Costs

There are many variables that will affect how much your extension will cost, such as:

  • 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. How Will Your Extension Project be Managed (and Who Will Do It)?

Who you have managing your extension project will affect how much your extension will cost. This is called your build route

Will you be the project manager for all or some of the project or will you hand this over to your main contractor, for example.

How you manage your project could mean a 40% variation from the cheapest route, based mainly on DIY, to the most expensive route, in which a main contractor takes on the role of project manager.

Homeowners usually choose one of four main routes:

  • 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. Get Accurate Quotes for Your Extension Costs

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.

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

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. Don’t Forget to Add VAT to the Cost of Your 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 tradesmen 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.

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