Basement impact assessment advice

What is a basement impact assessment?

A basement impact assessment is an independent professional engineering review of a basement proposal.  There is no common standard or requirements for basement impact assessments, although each Council has similar elements which they require to be appropriately addressed.

Typically, there are three general elements to a basement impact assessment. They should be addressed by three appropriately qualified and insured organisations:

  • The hydraulic civil engineering element – This should be undertaken by a professional Civil Engineering Consultant with specialist experience of hydrology, drainage, flooding, geology and foundations. It should review and propose mitigation in relation to the risk of flooding both to the basement owner and others.
  • The structural element – The proposed structural design and foundations should be undertaken by a professional Structural Engineering Consultant.
  • Practical construction elements – Short term practical issues regarding site access, removal of materials, noise and working hours. Often addressed by the Client preferred or appointed contractor.

 

Basements in London. Why basement impact assessments in London are becoming more common

Generally between 1800 and 1930, basements were commonly constructed in residential and commercial sites in London. However, they were often constructed without any waterproofing for the basic storage of coal or wine. They often had low ceilings and were excessively damp.  With the installation of mains gas in London and modern gas burning boilers, many of these spaces became largely redundant.

Basements are often expensive construction elements and have a high risk associated with their installation under an existing structure.  Many clients in London look to deepen their existing basement and upgrade to make the space usable. Other clients propose to demolish the existing house, install an extensive basement and then rebuild the house to a similar dimension, but to far higher standards.

As the value of land itself in London has increase, it has become increasingly cost effective. A client can have greater space, without the costs of moving and purchasing a larger and considerably more expensive house.  Although it can be a long and disruptive process, the savings for clients can be considerable and justify the basement proposal.

Basement developments in London 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.

Basements in London have been installed which have leaked. They have increase flooding, or caused structural problems for adjacent buildings. They have been undertaken without professional review and resulted in legal action and large claims for damage. As a consequence it is common for Councils in London to require basement impact assessments to show due-diligence of the proposal for both the applicant and public’s benefit.

 

Policy related to a basement impact assessment

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.

 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. Some Council’s in London are currently developing 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 (BIA)?

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

 

Cost of a basement impact assessment (BIA)

The extent of the proposals, the client expectations and risks involved can vary enormously. A basement within a flood plain for example is inherently at higher risk. The larger the basement, the greater the detail of assessment is likely.

It typically takes a few months for a basement impact assessment to be written and progressed.

Costs for a professional civil engineering BIA can vary between £1,500 and £20,000.

It would be appropriate to question any quote received below £1,500.  Even as a small professional consultancy with low overheads, we would find such a project would not be economically viable to be undertaken professionally. Always check they have the appropriate Civil Engineering Consultancy insurance with Lloyds of London and what the scope of the report is.

 

Our experience of basement impact assessments

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. 

This has included two of the richest people in the UK where the basement was three times the size of the house, right down to minor basement changes.

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 advice on a groundwater mitigation scheme for the development.

Wilsham Consulting Director Stephen Gibson has over 18 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 a basement impact assessment – how do I move it forward?

Please email –Wilsham’s Civil Engineering Director – Stephen Gibson  at stephen@wilsham.co.uk

Please include as many of the following elements in your BIA enquiry to aid our understanding of the work and time involved:

  • Existing and proposed plans,
  • Location plan and clear site address,
  • Your client address for the basement impact assessment
  • Any soil surveys, soil reports, boreholes, window samples
  • CCTV surveys of drainage and drainage plans.
  • Photos of the existing basement
  • Planning conditions or request from the Council for a BIA
  • Any experience of flooding externally or internally.

Fee proposals will exceed £800+VAT, or they would not be cost effective to undertake. A competitive fixed fee may be possible where there is a clear and fixed scope. No professional advice can be given prior to an agreed appointment.

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