Billionaire brothers David and Simon Reuben are planning a better-than- your-average refurbishment in London’s Mayfair following approval of their plans to convert a former social club into a £250 million (about €294 million) mansion. The brothers bought Cambridge House, on Piccadilly, in 2011, with plans to turn it into London’s finest family home.

This will involve a complete overhaul of the Grade I listed mansion, transforming it into a 45-room palace, complete with ballroom, an underground swimming complex and a wine cellar.

For many years it had been one of the world’s most exclusive gentlemen’s clubs, the In and Out, but the mansion fell into ruin over the past century. The property tycoon brothers (aged 72 and 69), born to Jewish-Iraqi parents, got planning approval after investing £5.5 million through the local Conservative authority towards an affordable housing scheme in Westminster. They initially offered £1.8 million but raised their offer before the planning committee granted permission.

The three-year project will transform Cambridge House into London’s most expensive home, and breathless speculation has already begun that the move will surely restore Mayfair to the top slot on the Monopoly board, after it was toppled by Kensington Palace Gardens in recent times.