Extending your home can be an attractive alternative to the upheaval of moving house, providing you with the extra space you desperately need and significantly adding to the value of your property. Before doing this, however, it’s good to have an idea of the costs involved in house extensions. Here’s a guide as to how much you might expect to invest in your extension in 2021.

Factors affecting cost

It’s impossible to give an exact figure as there are too many variables to consider, such as:

  • Location – Although a rough estimate per square metre (m2) can be calculated, the location of your home will make a difference. Homeowners in London and the South East should expect to pay about £20,000 more than those elsewhere.
  • Size – Obviously, the size will affect the cost. It is better to go for the largest you can afford rather than limit the size to focus on the interior. Extra space means extra value.
  • Type – Extending outwards is always more cost-effective than building upwards – if possible. Two-storey house extensions involve more complex work and will double the labour cost. Expect to pay about 30-50% more than a single storey extension.
  • Contractors – The better the contractor, the better the quality. There are plenty of reputable, reliable brickwork contractors and builders out there, be sure to check ratings and reviews before choosing one, and always get at least three quotes.
  • Use – If your extension is for a kitchen or bathroom you need to consider the extra cost of the electrics and plumbing. This will add at least £5,000 to the final cost of a bathroom, or £10,000 for a kitchen. Conservatories are probably the cheapest home extensions, averaging around £5,000 upwards.
  • Soil type – Clay soils can present a challenge to the gas/water supply, waste drainage and foundations, as well as contributing to problems later on. This will have to be taken into account by the contractors, which may well add to the cost.
  • Accessibility – Contractors will require clear access routes for the transport of tools/materials to and waste from the property.

Other considerations

Remember, you will need to add VAT (20%) to any quotes. Also, you will need to consider any possible ‘professional fees’, such as planning application, structural engineers, architects or building regulations, which could add another 10-15%.

Always check to see if any trees have preservation orders! Also, find out whether you need a ‘Party Wall Agreement’ with your neighbour (an extra £1,000 plus solicitors fees).


With these points in mind, you can find a rough estimate for your extension. For properties outside London/South East, expect a figure of between £1,200 to £1,500 per m2. Inside that area, it would be between £1,500 to £2,000 and above.

So, for something like a 5x5m single-storey extensions, you might get the following figures for the following sizes of extension –

  • Large £52 – £67,000
  • Medium £26 – £34,000
  • Small £16 – £21,000

These are without VAT or professional fees.