British Structural Waterproofing Association views building basements as a hazard, and they do not recommend the construction of such buildings especially where the water table is high.
If the water table is higher than the underside of the basement slab, this poses a significant risk as problems arise when the natural water level exerts pressure on the walls of the basement.
If the waterproofing is not well constructed, there are high chances of water penetration and leaks within the build. This will not only compromise the stability of the structure, but could also present a health hazard if the water is contaminated with wastewater.
The drainage and sewerage system of London is immense, having been redeveloped in the late 19th century. Although it has been extended as required over the last century, it is now undersized for the current high population and most of the system is over 100 years old.
This means the drainage systems are running at capacity in general day-to-day usage without adding rainwater, and with the amount of properties in London, the system is under constant strain. Without extensions for each new development, it can easily become overwhelmed.
This is common in areas such as Fulham and Chelsea, where generally the sewers are combined with foul and rainwater systems, so when heavy rains or floods occur, the sewer system receives more wastewater than it can accommodate.
This can lead to backfill, whereby all the lateral junctions of the system are filled to capacity and result in wastewater flowing back into private household drains.
If the property affected has a basement then gravity can cause the back-flowing sewage to leak from the drains, internal toilets, sinks, etc, and flood the basement with sewage and wastewater. The damage from drainage floods can be a hazard to health, very unpleasant and very costly to put right.