While American presidents live in the spacious White House, British prime ministers are expected to make do with a cramped upstairs apartment that used to be servants' quarters.
But in one of his first moves, Tony Blair, the newly elected prime minister, has indicated that 10 Downing Street is not large enough for his wife and three young children.
Instead, he intends to live next door at 11 Downing Street where Gordon Brown, his chancellor of the exchequer, would normally expect to live. Mr. Brown, a bachelor, is said by officials to be happy with the exchange.
By agreeing to swap houses, Mr. Blair and Brown, who are close personal friends, are breaking one of Britain's most enduring traditions. Prime ministers have lived at No. 10 since the early 18th century.
In doing so, they have put up with cramped quarters. No. 10 is less than a mile from Buckingham Palace, the 600-room home of Queen Elizabeth II. But in terms of living space, it is a universe away. Most of the house is used for official business, and little of it is available as living space for the prime minister's family.
Before the nation voted in the May 1 election, Blair said that he and his lawyer wife, Cherie, along with their three children, planned to live on the top floor of the house at No. 10.
But when the couple inspected the premises last Friday, they decided that even if it were remodeled, the new first family would be in a tight squeeze.
So the prime minister asked Brown whether he would surrender his much larger apartment.
Carol Thatcher, daughter of former Prime Minister Margaret Thatcher, who occupied No. 10 from 1979 to 1990, said yesterday that the house used to irk Mrs. Thatcher, especially in warm weather.
In accord with British political tradition, just-defeated Prime Minister John Major and his family have already vacated the premises. His wife, Norma, was known to escape No. 10 whenever possible. Friends say she found it too confining.
Tourists peering through the security barrier at the entrance of Downing Street might find that surprising. It looks to be a house of reasonable size.
But most of the space is taken up by official reception areas, a large room for Cabinet meetings, and a complex of offices. It is only at the attic level that visitors are confronted with a white door marked "private."
Behind it is a sitting room 15 feet by 25 feet, a dining room that seats only eight people, a minute kitchen, and four bedrooms, two of which are occupied by secretaries.
In theory, prime ministers could live at Chequers, an official residence in the countryside nearly two hours drive from London. But under the British parliamentary system, they have to regularly attend the House of Commons, sometimes at short notice.
The Palace of Westminster, where debates are held, is only a five-minute drive from No. 10 Downing Street.
American presidents and their wives have been heard to complain that the White House is a bit on the modest side. A few minutes behind the brick facade of No. 10 would lead them to count themselves fortunate.
It was not until Lloyd George took possession in 1916 that the house's first bathroom was installed. Today there are three.
James Callaghan, Britain's last Labour prime minister, who moved out in 1979, recalls, "Space inside is very tight. That is because it is really a set of offices with a flat [apartment] tacked on."
One of Blair's officials said the new prime minister would continue use the offices at No. 10, but "commute through the wall" of the adjoining townhouses.
Downing Street has not heard the voices of young children since the 1950s, when the late Prime Minister Harold Macmillan allowed his grandchildren the run of No. 10.