Ancestral abode of the Spencers -- Diana, Princess of Wales's family -- this great mansion is perhaps the finest example of 18th-century elegance, on a domestic scale, extant in London. Superlatively restored by Lord Rothschild, the house was built in 1766 for the first Earl Spencer, heir to the first Duchess of Marlborough. The gorgeous Doric facade, its pediment adorned with classical statues, makes immediately clear Earl Spencer's passion for the Grand Tour and the classical antiquities of the past. Inside, James "Athenian" Stuart decorated the gilded State Rooms, including the Painted Room, the first completely Neoclassic room in Europe. The most ostentatious part of the house (and the Spencers did not shrink from ostentation -- witness the £40,000 diamond shoe buckles the first countess proudly wore) is the florid bow window of the Palm Room: covered with stucco palm trees, it conjures up both ancient Palmyra and modern Miami Beach. COST: £6. Tube: Green Park.