Downing Street has four consecutive addresses, all on its north side. Number 9 is the Downing Street entrance to the Privy Council Office and now houses the Chief Whip's Office; Number 10 is the official residence of the Prime Minister and Number 12 now houses the Prime Minister's Press Office, Strategic Communications Unit and Information and Research Unit.
But it is Number 11, the official residence of the Chancellor of the Exchequer, which is perhaps the most authentic of the Downing Street buildings. Number 10 is the culmination of a knocking together of one of Downing's original 17th century houses with another, more impressive residence behind it which overlooked St James's Park (the architect William Kent linked these two in the 1730s). Number 12 on the other hand was completely rebuilt in the 1960s to a new design. So of the three houses, Number 11 probably gives the best impression of the scale of Downing's original speculative townhouses.
It is worth noting that the tradition which houses senior ministers in Downing Street dates back to a courtesy intended to the Treasury. Robert Walpole is now remembered as Britain's first prime minister, but he would never have used the title. He was First Lord of the Treasury - the title still emblazoned on the letterbox of Number 10 - and was offered Number 10 as a gift by King George II. Walpole eventually accepted only on the condition that it was not a personal residence, but was to be for the use of the First Lord of the Treasury.
As the role of prime minister developed, so the separate role of chancellor of the Exchequer grew. In 1828 Number 11 at last became the official residence of the Second Lord of the Treasury - and so the modern tradition of neighbouring chancellors and prime ministers was born.
It is also worth noting the distinction between the official residences and ministers? actual apartments, however. Throughout the history of these houses, ministers have lived by agreement in whatever rooms they thought necessary. During his last period in office, in 1881, Gladstone managed to claim residence in numbers 10, 11 and 12 for himself and his family - but then, he was both Chancellor and Prime Minister at the time.

