This astonishing planning document (above) illustrates why the London borough of Kensington and Chelsea has decided to outlaw the super-sized basement extensions that local billionaires seem so fond of. The multi-level subterranean recreation areas often go far beyond the boundaries of the aboveground properties, and have cracked walls and even affected the foundations of neighboring houses. In a famous case from 2012, excavation work under the mansion of a Goldmann Sachs director resulted in his neighbor getting trapped inside her home, unable to open her front door since it had shifted so much. A cabinet member for planning policy told The Guardian that basement extensions are "the single greatest planning concern our residents have expressed to us in living memory."

· Alas, No More Tricked-Out Basements for London Billionaires [Curbed National]

· All Ridiculous Amenity Alert posts [Curbed National]