<Tr> <Th> Designated </Th> <Td> 14 January 1970 </Td> </Tr> <Tr> <Th> Reference no . </Th> <Td> 1210759 </Td> </Tr> <P> 10 Downing Street, colloquially known in the United Kingdom as Number 10, is the headquarters of the Government of the United Kingdom 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 invariably since 1905, has been held by the Prime Minister . </P> <P> Situated in Downing Street in the City of Westminster, London, Number 10 is over 300 years old and contains approximately 100 rooms . A private residence occupies the third floor and there is a kitchen in the basement . The other floors contain offices and conference, reception, sitting and dining rooms where the Prime Minister works, and where government ministers, national leaders and foreign dignitaries are met and entertained . At the rear is an interior courtyard and a terrace overlooking a garden of 0.5 acres (2,000 m). Adjacent to St James's Park, Number 10 is near Buckingham Palace, the London residence of the British monarch, and the Palace of Westminster, the meeting place of both houses of parliament . </P>

When did 10 downing street became the prime minister's residence