Britain’s New Prime Minister Has Just Hours to Prepare

In the United States, a new president usually has a two-month window to prepare to move into the White House. Freshly elected British prime ministers move into Downing Street swiftly, often within hours of elections.

Keir Starmer, whose Labour Party won a commanding majority in the House of Commons, was formally appointed prime minister on Friday by King Charles III. Just hours after his predecessor, Rishi Sunak, made his exit, the new prime minister is about to make a speech to supporters outside the front door of 10 Downing Street.

He will be ushered inside the building by staff members and receive a briefing from members of the civil service that, according to the Institute for Government, covers topics including living arrangements, security protocols and appointing ministers.

Number 10 has had a great deal of practice with quick transitions. Since the last general election five years ago, three prime ministers have lived there, including Liz Truss, who called Downing Street home for less than seven weeks.

The address has been home to Britain’s prime ministers since 1735. It is much larger than it seems from the street. Beyond the much photographed black door and the checkered entryway hall, offices, living areas and reception rooms stretch out to much of 12 Downing Street. A corridor connects to Number 11, the official residence of the finance minister, or chancellor of the Exchequer.

Prime ministers often put their own stamp on the living quarters, but that mostly remains out of public view. When Margaret Thatcher won her first general election and moved into Downing Street in 1979 — she called it “living over the shop” — she gave the place a makeover.

“The first-floor study was redecorated to her taste,” wrote the historian Anthony Seldon, describing how she chose a light gray wallpaper, cream-colored furniture and modern art.

“She was intensely proud of her study: on one occasion a guest spilled coffee over the settee and carpet,” he wrote. “Typically, Mrs. Thatcher spent 20 minutes herself trying to remove any trace of a stain.”

In 1997, when Tony Blair became prime minister, he and his wife Cherie chose to live in Number 11, rather than Number 10, because of its larger living area. Their fourth child was born while they were living there.

In 2011, when Michelle Obama visited, the world got a glimpse of how David Cameron’s wife, Samantha Cameron, had redecorated the place through a photo released by the White House that was pored over by the British press.

Ten years later, Boris Johnson’s then-fiancée, Carrie Symonds, put her own lavish stamp on the residence, sparking an investigation. The prime minister is given an annual allowance of 30,000 pounds, or around $38,000, from the public purse to pay for renovations to Downing Street. The renovation reportedly far exceeded that amount.

Its occupants have not always been comfortable. Ms. Truss said in her memoir that she had spent weeks being bitten by fleas that some suspected had been brought in by the dog belonging to her predecessor, Mr. Johnson. Number 10 had to be fumigated.