Farook Owadally, 55, bought the property – a shop with a maisonette above – in Maida Vale, London, for £965,000 in 2008 and converted the entire property, without planning permission, from its basement to its third floor into flats, some of which were just one small room.
Over ten years, he made huge profits by charging the tenants in each flat monthly rents of £1,000, said the Westminster Council.
Several flats did not meet minimum size requirements, and one was just 18 square metres.
This month, after a prolonged investigation, at Southwark Crown Court, Owadally was ordered to pay a £400,000 confiscation order under the Proceeds of Crime Act plus a £90,000 fine and £40,000 costs.
Westminster Council said it was the largest such fine it had ever imposed and said that Owadally had ignored warnings over a period of seven years, with enforcement notices dating back to 2012.
Owadilly has been given three months to pay the confiscation order and six months to pay the fine and costs.