Downing Street is in Whitehall, London, near the Houses of Parliament. For the past 200 years the street has been the official residence of the British Prime Minister and the Chancellor of Exchequer. Traditionally, the Prime Minister's official residence is No. 10 Downing Street and the Chancellor's official residence is No. 11.
No one lives in No. 12, it is used as offices for, among others, the Prime Minister's Press Officer.