10 costs you need to consider for your basement conversion before jumping in feet first

Firstly each project is different.

In some cases, creating a space underneath the existing building is the only way to increase the usable square footage of the building – one reason that basement conversions are so popular in central London, where opportunities for above-ground alterations are minimal.

Besides adding usable space to your home, a basement conversion can help to significantly increase the value of your house.

Depending on the area, factors like access, available headroom, and the potential for natural light can significantly affect the value per square metre of a basement conversion.

In some areas, however, there is considered to be a ‘ceiling’ value for property prices that, no matter how beautifully converted your basement, is hard to surpass.

It can be worth consulting a local estate agent for their opinion before making hard and fast decisions regarding the nature of the basement development to undertake.

It is worth noting, however, that many London authorities now limit retro-fit basements to a single storey beneath a property and a maximum of 50% of the rear gardens. This was to stop so-called “Iceberg” basements in the most expensive areas of the capital.

Factors affecting cost

A basement conversion will always work out cheaper if there is no excavation required – either to create the underground space from scratch or increase the headroom in an existing basement. Difficulties with excavation can include:

  • concrete oversite slabs at the ground floor
  • sewer and drain diversion
  • difficult ground conditions, e.g. rock, clay, sand, peat, marsh
  • a high water table
  • access for removing excavated materials

Costs for basement work can vary significantly depending on the jobs to be undertaken

An idea of the costs involved:

  • 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 lightwell / external access: £5,000 – £7,500
  • Engineer’s fees: £1,500 – £2,000 plus a percentage of the contract sum
  • 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. Leasehold properties could be subject to party wall awards for each level – groundfloor, first floor, second floor and so on
  • VAT: an additional 20% added by VAT-registered contractors

Before you embark on your basement project ensure you take all of these factors into consideration to make your investment worthwhile.

For more information on the construction of basements to meet our warranty requirements, download Section 2 of the LABC Warranty technical manual.

Also take a look at ASUC’s guidelines on basement construction, issued in 2013 and revised in 2016.


Please Note: Every care was taken to ensure the information in this article was correct at the time of publication. Any written guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.

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