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Expanding downwards Developing basements can cause a headache for developers, says Phil Slater

Developers and housebuilders working in London who are planning to extend their projects downwards by developing basements may find their schemes thwarted, unless they address new regulations introduced earlier this year by Camden Council.


ost applications for basement developments or extensions now

require a Basement Impact Assessment (BIA), as they include assessments of groundwater flow, surface water flooding and slope stability. Let me explain why. London clay, on which most of

north London sits, is particularly sensitive to changes in natural moisture content. Consequently, basement developments and associated activities, such as tree felling or changes to natural drainage patterns, may have significant impacts, including subsidence and ground heave. Understandably, Camden

Phil Slater is a hydrologist at SLR Consulting, the international environmental consultancy.

Council has decided that the potential for possible catastrophes should be properly assessed before construction work begins. In fact, before the new regulations were introduced, one project in Kings Cross, where recommendations to a basement developer to carry out modelling for risks to neighbouring properties were ignored, resulted in major structural damage. More

significantly, there have also been a number of builders who have died in the capital as a result of structural collapse during basement excavations. The new Camden BIA means

that planning applications for basement extensions are evaluated on whether they will cause harm to the built and natural environment. And, by ensuring that the critical areas of concern are appropriately screened at the outset by professionals, including hydrogeologists and geotechnical engineers, the independence of the necessary studies is assured. Haringey’s Local Development Framework also requires similar assessments for basement developments. I wouldn’t be surprised if, eventually, such regulations become nationwide. If potential problems are

identified, it needn’t mean the end of plans for subterranean expansion. Further stages of site investigation and impact assessment may be necessary and may result in the introduction of mitigation measures such as additional drainage, waterproof ‘tanking’, support to neighbouring structures. These assessments are needed

for safety. They are also a brilliant way of allowing development below ground level, making houses more attractive to potential buyers. 

Chartered Geologist Phil Slater of SLR Consulting has 15 years’ experience as a hydrologist. His areas of expertise include groundwater modelling and risk assessments, impact assessments for planning applications, quarry dewatering, water supply and the growing area of basement impact assessments. He is also interested in ground source heating and cooling. Prior to joining SLR, Phil worked as a regulator for the Environment Agency. SLR is an international environmental consultancy with almost 900 employees working in 63 offices in Europe, North America, Australasia and Africa.

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