How much does an extension cost? That’s the first question that would-be extenders ask as it is often the deciding factor on whether to take the project on or not.

The cost of your extension depends on a number of factors — the build route you want to take, the number of storeys you’re going to build and remembering to add on costs like VAT.

In order to take control of your budget from the outset, make sure you are aware of these factors that will all impact the cost of your extension.

1. What Affects the Cost of an Extension?

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
  • 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 per cent more than the cheapest locations.

Depending where you are in the UK, for a straightforward extension you should allow around £1,000–£2,000/m².

Remember, too, that 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.

2. How Will You Build the Extension?

Your build route is based on how you want to manage your extension project and will affect how much your extension will cost.

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

3. How Much Does an Extension Cost to Build?

 A single-storey extension will cost roughly the following:

  • Basic quality: £1,000 to £1,680/m²
  • Good quality: £1,680 to £1,920/m²
  • Excellent quality: £1,920 to £2,160/m²

And the cost of a two-storey extension will cost 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.

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.

Remember, balance the amount you are willing to spend on your extension with the estimated value it will add to your home.

How much does an extension cost

Including a glass link to connect an extension to an original house will have an affect on how much your extension will cost as will installing aluminium bi fold doors, like the example above from IQ Glass UK.

4. Quotes and Estimates – What’s the Difference?

It sounds obvious, but not being clear about the difference 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 Need 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 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.

Calculate How Much Your Extension Will Cost

This free extension cost calculator will estimate how much your extension project is likely to cost, based on the answers to questions like where you live, the kind of extension you are interested in, how much of the work you will do yourself and the standard of work (standard, good or excellent).

You’ll get an instant, accurate result as to how much your finished project should cost for a range of projects, including:

  • single and two-storey extensions
  • loft, cellar and garage conversions
  • basement extensions.

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