10 Downing Street, colloquially known in the United Kingdom as "Number 10", is the headquarters of the executive branch of the British Government and the official residence and office of the First Lord of the Treasury, a post which, for much of the 18th and 19th centuries and ever since 1905, is also held by the Prime Minister.Situated in Downing Street in the City of Westminster, London, Number 10 is over three hundred years old and contains approximately one hundred rooms.