The Financial Services Authority is taking two extra floors in London's Canary Wharf tower to house staff recruited after the financial crisis.
The regulator, which will be scrapped if the Tories win this year's election, has overflowed from its building in North Colonnade at the base of the Docklands tower following a U-turn on reducing its headcount as it monitors firms more closely.
The FSA board has taken leases that expire in 2018, the same time as its main block. It currently spends £48m a year on accommodation.
Under a programme called "The FSA in 2010" drawn up in 2006, the regulator proposed reducing its staffing levels as it implemented a light-touch policy. John Tiner, then its chief executive, said in that year's business plan: "I anticipate that we will be able to meet our strategic aims with fewer people than we currently employ."
Following criticism of the light-touch policy, the regulator has reversed the staffing decision. The salaries budget for 2009-10 has been increased by 24 per cent to £306m.