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How Much Does An Extension Cost?

In short, most extension projects cost around £1750-£2750/m² of new internal space. So a 30m² kitchen extension could be estimated at somewhere between £40,500-£67,500, plus, VAT at 20%. 

If that sounds suitably general — that’s because it is. And that’s all you can expect when you have no detail or idea of what the extension looks like, or what materials you’re going to use, or how it is going to be built (and who’s going to do it). The £1750-£2750 price, however, is a very good range of pricing based on averages.

There are multiple factors that can affect your project’s costs, including:

  • How many storeys you’re going to build 

  • The size and shape 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

  • Whether the extension contains a kitchen or bathroom 

There are other costs to consider outside of the build itself. 

Here are some example costs you might need to factor in:

  • architect fees (check out our instant fees)

  • structural engineer (£500-£1,250) 

  • surveys (between £700-£1,800)

  • planning permission (if needed — currently £206 in England for a two-storey extension, but check with your local authority).

  • Building Regulations applications

  • Lawful Development Certificate (currently £103)

  • fitting out the extension (costs will vary depending on how you plan on using the new space)

  • Party Wall Agreement (if needed — £1,000-£2,000 per affected neighbour for a straightforward agreement outside London).

  • insurance

The overall quality of the extension will impact the estimated cost of the project, the costs quoted below are for an insulated cavity wall extension with 100mm blockwork and faced with brick. 

A standard quality extension represents most speculative developers concrete interlocking roof tiles, studwork partitions, contract kitchen and radiator central heating. 

Good quality extensions also use better quality facing bricks clay roof tiles, high-end softwood joinery, blockwork partition walls, a top-of-the-range kitchen and underfloor heating. 

An excellent quality extension sees top-of-the-line facing bricks, play clay tile roofs, hardwood joinery, a bespoke kitchen, high quality sanitary wear and underfloor heating.  Standard quality: £1750 to £2000/m²

Good quality: £2000 to £2500/m²

Excellent quality: £2500 to £2750/m² or more.

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

NOTE: The indicative figures used in this calculator were last updated in 2023 and do not include the cost of the site, professional fees, external works or any VAT.

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