Basement impact assessments and groundwater

Land values are generally going up in central London and starting to rise in the suburbs. With development space is severely limited, there is intense commercial pressure to ensure that all development maximises usable space.  Basements are often expensive construction elements. They can often only be justified when usable space is at a premium.

Basement developments typically have an impact on local flooding, groundwater flow and water quality and can cause ground movements. All these issues could impact on immediate neighbouring properties, with or without basements.

Policy related to basement impact assessments

The London Borough of Camden adopted CPG4 in April 2011. This is a five stage process in support of planning guidance to use in the consideration of planning applications involving basements in the Borough.

Basement impact assessment

 This planning guidance is a formal and structured approach to identifying any potential risks to the development and the locality. CPG4 covers the screening of the applicant and actions up to the review and decision of the planning office. Other Council’s in London are currently developing their own basement impact assessment standards.



In Camden, where the site of a proposed development include a new or extended basement a it has a general statement on meeting the terms of Camden Development Policy 27.

  •  Impact of proposals on surface flow and flooding
  • Impact of proposals on groundwater flow
  • Impact of proposals on structural stability

So what is a basement impact assessment?

A basement impact assessment is a process of incorporating a combination of surface and groundwater hydrological conditions, with geotechnical analysis into a comprehensive review. The objective is to establish the risk of ground movements arising from the basement development and any impact on adjoining or neighbouring properties and the potential flooding risk.

When submitting applications for basement developments, the level of technical information required for a basement impact assessment will vary according to the type and size of the development. The following may be required for a full basement impact assessment:

  • Desktop study of existing geological and hydrological conditions of the site and the wider area. This is in order to identify areas susceptible to instability (ground and water movement) and localised flooding, needs to be site specific
  • Detailed engineering study undertaken by a professionally qualified and experienced civil engineer/hydrological geologist to assess local ground conditions, water movement, subsidence and drainage including through the use of boreholes, potential impacts on adjoining/nearby properties
  • Identify suitable construction methods and mitigation measures for developments which may affect the stability of the host and neighbouring buildings and/or nearby structures, and hydrology (at the site and within the area), without placing additional pressure on other areas or on the local combined sewer network
  • Devise a method for monitoring local ground conditions, water movement, subsidence and drainage

Wilsham Consulting Ltd, as one of the leading small civil engineering consultancies in the UK has seen a steady increase since 2011 in requests for basement impact assessments. We have completed or reviewed many basement impact assessments for numerous multimillion pound private developments. 

What we have found which is of concern is that we are often brought in to advise the contractor once work has already started on site and a considerable groundwater problem has been encountered. Our key role then is to design or advise on a groundwater mitigation scheme for the development.

Wilsham Consulting Director Stephen Gibson has over 17 years’ experience of working with Local Authorities and the Environment Agency. We understand that clients often need practical cost effective solutions, while meeting best practice standards.  It should be noted that we don’t write reports to tick boxes or suggest we have any magic wands such that an Authority will agree swiftly. Our reports remain independent professional assessments and in doing so maintain our professional integrity in the industry and with the Authorities.

I want professional advice on basement impact assessments – how do I move it forward?

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You can:

yesSpeak to Wilsham Consulting Director – Stephen Gibson directly about your project on 01235 529646

mailEmail your proposals directly to Stephen Gibson at

 – Please include all relevant background information for a fixed quote, including plans, location, surveys, photos, planning conditions, or anything else relevant to the your proposals.

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