Beneath the games room of a £15 million house in Hampstead is the concrete shell of a swimming pool that the current owners have never fully installed. They had it built four years ago, at the same time as the basement gym, car showroom and cinema, in case a future buyer insisted on having somewhere to take a dip – a good call, given that around 400 mansions in London now have subterranean pools. Yet there’s a chance that this pool could remain permanently hidden beneath temporary flooring. New analysis of council data by law firm Boodle Hatfield suggests that the trend for ‘iceberg’ homes – those with lavish, mega-storey basement extensions that contain underground banqueting halls or car museums, and are often significantly larger than the actual house above ground – is on the wane. Applications for basement excavations in Westminster fell by 27 per cent last year – 99 in 2019, compared with 136 in 2018....