Basements Extensions: A Look at Costs and Options in 2022

Update 2022: The cost of major home improvements has soared over the last 18 months, this has been widely reported in the media and confirmed by our own research. The basement extension prices per square metre on this page were updated in 2022 but as the cost of materials is so volatile they are subject to change. If you would like a fixed price, fill out this form with details of your project.

Basements are a great way to increase the value of a home and add extra living space. While basements aren’t cheap, when you consider the cost of moving home and rising house prices, this type of extension is still attractive.

Everything you need to know about basement extension prices can be found on this page.

But first…

How Much Do You Think a Basement Extension Will Cost?

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The question below is entirely optional but answer it and you can see how others voted:

How much do you think a 25 square metre basement extension will cost?

Factors That Will Affect The Price

Every basement extension is different and no two projects are ever the same.

The price for one extension could work out quite different from a similar one, even one completed on the same street.

Here are the key points to consider:

  • Converting an existing cellar will cost around half as much as digging out an entirely new basement. If you have an existing cellar, you could get extra usable floor space at a great price.
  • Basements underneath the garden are cheaper than those directly under the house.
  • Creating external access to the basement will add around £6000 – £8000 to the cost of the basement.
  • The home’s proximity to the water table is a key pricing factor.
  • Diverting sewers and drains can be very costly.
  • General access issues and problematic soil types can also bump up the price.

Our Basement Extension Cost Guide

In early 2022, we got in touch with six basement extension companies based in the south-east of the UK and asked them for an estimate to create a basement beneath our 3-bed property.

We didn’t want a swimming pool, jacuzzi or anything like that, just a room that we could use as a second lounge.

The figures below are the average of the prices given to us.

The average price for a basement shell; that’s the dig out, underpinning and creation of the concrete structure, will cost around £3750 per square metre.

The fitting out of the basement, that’s the electrics, plumbing, staircase, plasterboard walls and flooring etc will depend on the desired specification but we were told to budget from £750 per square metre for a very basic fit-out.

What’s Not Included in These Figures

Architect and structural engineer’s fees, survey fees, party wall fees, planning application and building regulation application fees are not included in the prices shown above. These costs shouldn’t be underestimated as they can add up to a considerable amount so we suggest you budget for:

Architect fees are usually between 5% and 15% of the overall build cost.

Structural engineer fees vary from project to project but around £1100 appears to be the going rate for a basement of this size.

A planning application isn’t always needed but will cost upward of £175 if you need one.

Building Regulations approval can cost from £700 but varies.

Party wall agreements can cost up from £500 up to £1500 per neighbour.

Survey fees for drains, water table, trees and general geology will vary but often cost several hundred each.

Our prices also exclude the cost of kitchens, bathrooms and other high-end extras that you may require.


The cost of basement extensions has gone up considerably since the start of the pandemic, the cost of some materials is 30-50% more than they were only two years ago and a shortage of skilled labour and plenty of demand has pushed prices higher.

In 2022 we think a decent basement extension will cost from £4500 per square metre with professional fees on top.

Of course, the prices on this page are also specific to the basement we wanted constructing under our property, your project could be completely different.

As prices are very volatile at the moment and they could go up or down, we urge customers to get a fixed quote by filling out this form with details of their project.

Have you considered a garden basement extension? By digging out the garden, rather than the space under the property, you could save money as there’s less underpinning. Prices vary but are usually from £3000 per square metre still.


Compare Basement Prices to Extensions and Loft or Garage Conversions

We know from experience that garage conversions are the cheapest and easiest way to create additional usable floor space in the home. This is because the structure is already built and just requires an upgrade and fitting out. Explore our garage conversion prices here.

The price for a loft conversion will vary and depends on the size of the build and also whether roof windows or dormers will be used. While prices start at around £27,000 we think you’ll pay closer to £35,000 for a fully fitted loft conversion with a dormer; add a bathroom and the price goes up further. Explore our loft conversion guide here.

For single and double-storey extensions, check out our house extension cost guide.

Get a Quote

If you would like a quote for a basement extension, just tap the button below and fill out the form:

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In our experience, a small conversion can take as little as 1 - 2 months while a project involving a full excavation can be completed in around 3 - 4 months. A larger and more complex project takes between 5 and 7 months.
Not all extensions are created equal. You can maximise your investment by creating extra bedrooms (with en-suite if possible), extending kitchens and creating kitchen diners. Extra space in the home without any practical uses is less likely to increase the value of the property.
The government allows homeowners and developers to extend a property without planning permission provided the work is completed within a set of rules covering both design and location. This is called permitted developments and is a great way to extend your home without the cost and hassle of red tape.
Loft extensions and integrated garage conversions are the easiest and cheapest way to add extra living space because you're not increasing the footprint of the existing building. Side or rear single storey extensions are more expensive while double storey extensions and basements are the most expensive option.

This guide was written by and was last updated in May 2022.

Don’t forget; we have price guides for hundreds of home improvement projects.

Explore our full list of detailed price guides here.

Author - Danny Woodley
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