The build route you want to take, the number of storeys you’re going to build and remembering to add on costs like VAT – these are all factors that will affect how much your extension will cost.

We’ve put together five top tips to help you get a real sense of how much an extension might cost in order to set your budget from the outset.

1. Think About How Your Specification Can Affect Your Costs

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

 single-storey extension will cost roughly:

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

The cost of a two-storey extension will be 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.

(MORE: Building an extension: a beginner’s guide)

2. What Factors Can Affect the Cost of my 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.

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.

Use this free calculator tool to estimate how much your extension project is likely to cost.

3. How Your Build Route Will Affect How Much Your Extension Will Cost

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
contemporary extension

 Read more about the transformation of this granite farmhouse. Image: Nigel Rigden

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: Rear extension design ideas)

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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