What about flooding?
Basement works that need excavation are now subject to planning permission, and if your house is in a high risk flood area, you may well have a problem. This is not because of water welling up from below, though. It’s because if the nearest river overflows, then surface water in the streets would flood into the basement. This is more of an issue where a basement dwelling is a self-contained, separate unit, because the occupier can’t just escape upstairs within the house. Therefore, one of the aspects to look at and to design for, is the drainage capability of the new basement.
What about rising groundwater?
For some time, press reports have claimed that London’s groundwater levels are rising and that basements could become subject to flooding because of this. But is there any evidence?
London’s groundwater is held mostly in chalk aquifers. Originally, the capital’s groundwater was at much higher levels than it is now, but massive removal of the water in the nineteenth century as the population of London increased, made the level drop considerably, and London turned to its rivers to source its drinking water. So the groundwater rose again. Following strategies to manage the water level, Dr. Michael Jones, of Thames Water, has stated recently that analysis of groundwater levels and reduced levels in high risk parts of London both indicate that the GARDIT Strategy has worked to control rising groundwater levels in central London. See the source here: https://www.groundwateruk.org/Rising_Groundwater_in_Central_London.aspx.
It’s safe, therefore, to assume that rising groundwater is probably not something that needs to keep you awake at night.
What about structural instability?
It is essential that you use a company that has professional expertise and experience in building basement conversions. A basement conversion in London is not a job for your builder’s brother, who just happens to have a digger. Consider, for example, what happened recently to a £1m property in Kingston. Its owners instructed builders to start digging out an extension. They hadn’t spoken to the council, or gone anywhere near building control, and they clearly weren’t using a professional basement conversion company. In the middle of the night, the entire five bedroomed house collapsed into the excavated hole. All that is left is a heap of rubble.
That’s why professional companies use structural engineers and go through building control. Let’s hope for the homeowner’s sake that the construction company was fully insured, but it’s doubtful.
London has unrivalled expertise in this field of building, and as commentators have pointed out – if rising groundwater and structural collapse and all the rest of it, were probable, we wouldn’t be digging out Crossrail, would we?
So use a professional and experienced company, that you can trust to be careful and conscientious. Membership of the Considerate Constructors’ scheme is a good start and wins extra points with the council on the planning application.