London Basements and Flood Risk

With London property prices at some of the highest values in the world it is no surprise that owners often desire to maximise the space that they have. Undeveloped available land alongside an existing home is virtually non existent, and even if it were, planning permission for an extension can be a huge obstacle. The same applies for extending upwards. Attics have already been converted, and with the desire to continue extending then the only option is down. Basement extensions are fast becoming a must-have asset to properties in London, particularly those at the higher end of the market.

Historically, basements were built to provide a space for storage that was cool, safe, and out of the way. In the 21st Century fridge-freezers do most of the job that cellars were originally intended for. The current trend in basement developments has an altogether new purpose and style. No longer do people simply want a dark and dusty hole under their house; they want extensive and luxurious leisure facilities, with swimming pools and games rooms.

Over the past few months Haycock have reviewed a range of plans for basement extensions, including multilevel basement developments with room for car parking, spas, staff quarters and even double storey cinemas. If they were to be approved by planning authorities then some of these properties would become the most valuable private houses in Britain.

Despite their subterranean nature, basement extensions are still subject to planning consultation; objections can be made, and refusal can be given. This may come as a surprise to some people as it is reasonable to assume that the construction of a basement is "out of sight", "out of mind" and therefore permitted without question. However, concerns have been raised regarding the impact that a basement may have on local hydrology, particularly with groundwater flows in mind. Haycock have carried out reports and investigations into the suitability of several basement extension proposals in the area of Highgate and Hampstead Heath, London on behalf of the Local Authorities and concerned residents. Haycock have been researching the potential impacts that may be caused by the construction of large basement developments. Although we do not inherently object to these proposals, it has become apparent that there are some significant issues that need to be addressed when considering such a development.

Traditionally the concern is on the behalf of the developer, who wishes to ensure that their basement is watertight so that they do not suffer from damp, or even flooding. It is therefore common to "tank" a basement construction, so that it is impermeable to groundwater intrusion. The problem comes when you consider what that newly installed area of impermeable space will do to the previously existing hydrogeological regime. One recent proposal for a basement extension in London had a volume of underground space of 22,000m3 that would accommodate a luxury leisure and living complex. The removal of that much subsurface material, replaced by an impermeable void means that it is likely to have knock-on side effects to the way groundwater behaves within that region.

Such side effects include heightening the risk of groundwater flooding to neighbouring properties, due to displacement, and potential subsidence caused by continued pumping of groundwater to maintain a basement's dryness. Another less obvious risk is the potential impact that a basement construction can have on nearby water bodies. An example that Haycock have come across in recent months is of an application for a basement construction that is located within close proximity of Hampstead Heath and its famous chain of lakes and bathing ponds. If the construction of this large basement were to go ahead then our modelling shows that there is strong potential for it to divert underground streams off their natural course with a consequent loss in water volumes to some of the ponds. 

With the rise in planning applications for major basement constructions in London and other cities, Local Authorities have found that a review of the planning policies to these schemes is needed. It is becoming widely apparent that the potential for detrimental impacts caused by basement construction is a very real issue. In many examples basement extensions can be successful additions to properties. However, as we have discovered, applications can fail due to the potential for the disruption of the natural groundwater regimes, and therefore it is advised that this issue receives attention at the early stages of any development proposal.