John Caudwell, the man who made more than £1 billion from Phones 4U, has been given permission to start work on an enormous Mayfair mega-mansion - including an incredible 14,999 square foot underground extension.
So what is he doing, and how unusual is this sort of project?
The plansWestminster Council has given 61-year-old Caudwell the go-ahead to turn two Mayfair houses into one enormous mansion, complete with an enormous underground extension. The houses are back-to-back at the moment, so the extension will link the two by tunnelling underneath their gardens.
Caudwell bought the houses for £81 million in 2012, and at the moment he and his family have a flat at the top of one of them - although most of the time they live in a 50-bedroom Jacobean mansion in Staffordshire.
According to Bloomberg, the finished property will be around five times the size of a normal Mayfair mansion - and ten times the size of a normal family home. The Daily Mail claims it will also be worth an estimated £250 million.
The underground section alone will be home to everything from a pool and gym to a sauna and salon, games room and car park. There will be two underground bedrooms and ten above ground in the existing properties.
PopularUnderground extensions are increasingly common in central London. The borough of Westminster alone has given permission for 181 in the financial year from 2011 to 2012, although there have been reports that the council is planning to get tougher on those planning this kind of building work. The homes they create - where the vast majority of each home is beneath the surface - have been nicknamed iceberg homes.
It's hardly surprising they are so popular. After-all there's only so much space in central London, and if you can't go up or out any further, the only other option for adding to your property is to dig down.
Amongst the celebrities that have embraced this approach are Tamara Ecclestone (who added a bowling alley, swimming pool, disco and cinema), Lakshmi Mittal (who added a jewelled swimming pool and steam rooms), Ricky Gervais (who built a leisure suite), Andrew Lloyd Webber (who almost doubled the size of his property) and David Cameron (who created more space for his family).
However, not every celebrity is a big fan of the 'iceberg' home. This week Joan Collins spoke out against them, saying that she was saddened by the amount of building work going on around her Belgravia home.
But what do you think? Would you want to live underground?