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How much does a Basement Conversion cost?

Peter Little | CEO

Published | 3rd Apr 2017


The cost of converting or excavating a basement depends on several factors but could be more affordable than you think…

Converting a basement, although not always cheap, is a fantastic way to add value to your property while increasing your living space considerably. It is the ideal solution if you need to upsize but don’t want to move home ­or you can’t afford to buy a bigger property in the same area – in fact, when you consider rising house prices and the cost of moving home these days, the cost of a basement conversion can actually seems like good value!

There are several factors to consider when pricing a basement conversion as detailed below:

Existing cellars

If there is an existing basement or useable cellar space, and you don’t need to do anything structural, the cost of renovating could be around £1,500 per square metre. However, once you start having to increase head heights and lower floors, even within an existing space, costs start to shoot up.

Basement excavation

A completely new basement excavation will always be more expensive simply because of the amount of work involved. As well as the cost of removing excess materials, there is the cost of tanking or damp proofing the basement, underpinning walls, rerouting sewers or drains, as well as any problems relating to the area’s topography, such as overcoming difficult ground conditions or high water tables. The costs for excavating a new basement will be somewhere between £2,500-£4,000 per square metre.

Basement use

Another factor to consider is how the room will be used – popular uses for basements include media rooms and home gyms which don’t require much in the way of technical décor. For newly excavated basements that will be used in this way you will be looking around the £2,500 per square metre mark. However, for higher spec basement rooms, such as state of the art kitchens or underground swimming pools which require more complicated technical procedures, the cost will be at the top end, around the £4,000 per square metre mark.

Additional extras

On top of these basement build costs you have all the extras to consider – planning applications, party wall agreements, building regulations, and whether you require an external access to your new basement conversion.

Before getting bogged down in all the detail, it is best to ask for a free quote so you know what your options are. Cool Basements offers free advice and a free initial consultation if you’re thinking about a basement conversion. We can talk you through the options and try to suggest a solution that meets your budget.

To help with your planning, here is the Basement Waterproofing Association’s guide to costs for basement conversions:

  • Conversion of an existing cellar: £750 – £1,400 per square metre
  • Lowering the floor level and underpinning an existing cellar: £1,500 – £2,000 per square metre
  • Digging a new basement space and underpinning: £2,000 – £3,000 per square metre
  • Digging a new basement space underneath a garden: £1,500 – £2,000 per square metre
  • Creating a light well / external access: £5,000 – £7,500
  • Engineer’s fees: £1,000 – £1,500
  • Planning application, if required: £165
  • Building Regulations application: £750+ depending on the value of the work undertaken
  • Party wall agreements, if required: £700 per neighbour
  • VAT: an additional 20% added by VAT-registered contractors

Last updated 2021. Any costs above are purely ballpark estimates.

Peter Little | CEO

Published | 3rd Apr 2017

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