Basements – A Look at Costs and Options

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 now looks more and more attractive.

There are dozens of ways to add extra floor space to a home, garage conversions are often the cheapest with loft extensions only slightly more expensive.

We’ve already explored house extension costs and on this page, we’ll look at basements and how much they cost.

How Much Do You Think a Basement Extension Will Cost?

We like to ask our visitors how much they think home improvement projects will cost.

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

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 with a small bar.

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 £2600 per square metre +VAT.

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 £900 per square metre +VAT for a basic fit-out.

For a 25 square metre basement, the build cost came to £87,500 +VAT.

That’s a whopping £3500 per square metre for a basement extension fitted out to a basic standard.

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 cost.

These can add up to a considerable amount so we suggest you budget for:

Architect fees are usually between 9% 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.

Party wall agreements can cost up to £750 per neighbour.

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

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 £25,000 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 2021.

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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