The house which became a Palace
At the termination of the Mall sits Buckingham Palace. The Palace was originally built as a townhouse for the Duke of Buckingham in 1703, and was later acquired by King George III in 1761 as a private residence for his wife, Queen Charlotte. The palace underwent several expansions and renovations over the years, most notably by architects John Nash and Edward Blore in the 19th century, and Aston Webb in the early 20th century. The Palace has an incredible  775 rooms, including state rooms, guest rooms, offices, and staff quarters. It operates a bit like a small village with its own post office and even cash machine.
 The Palace is also surrounded by the largest private garden in London. Buckingham Palace is the official London residence and administrative headquarters of the British monarch.
 
Interesting facts
The palace operates with a level of autonomy supported by several surprising amenities. It contains its own doctor’s surgery, a post office run by Royal Mail, a cinema, and even a private ATM in the basement provided by the bank Coutts. Additionally, there is a swimming pool, installed in 1938 on the north side of the building, which was famously used by the late Queen Elizabeth II and her sister to learn to swim.
Persistent reports suggest the existence of a network of underground tunnels that potentially connect the palace to nearby landmarks, such as the Houses of Parliament and Clarence House. While these remain largely unconfirmed by official records, lore persists about their use during times of war or political unrest. One famous—if apocryphal—anecdote claims that King George VI and the Queen Mother once encountered a man from Newcastle living within these passageways while they were exploring.