An esteemed Manhattan firm responsible for the New York Public Library and many society mansions designed the seventy-seven-room Nemours house. Presumably, du Pont knew the firm from his frequent vacations in Florida, where they had built Whitehall in Palm Beach (1901) for tycoon Henry M. Flagler. Nemours was one of Carrère's last projects, as he was killed by a taxicab in 1911. Thought to have cost a then-astonishing $2 million, the 102-room home derives from the French chateaux as perfected by eighteenth-century architects A.-J. Gabriel, Alexandre Théodore Brongniart, and Claude-Nicolas Ledoux. It shows similarities to Carrère and Hastings's Vernon Court, Newport, Rhode Island (1901), including a decorative external latticework on one side (the firm's Frick Mansion, New York, of 1913–1914, also had such latticework, removed in the 1930s). The coupled Corinthian columns of the loggia were a favorite device of McKim, Mead and White, with whom Carrère and Hastings had trained. Nemours was built of Nicholson, Pennsylvania, blue stone plus clay tile and concrete, and it has an exterior finish of stucco and Indiana limestone. A third-floor balustraded deck at the foot of the hipped roof affords views of the garden. The contractor was James M. Smyth of Wilmington, with whom du Pont had a famously close and trusting relationship, as indeed he did with Hastings.
Drawings and blueprints survive for a never-executed Gothic Revival chapel connected by a cloister to the house (1920, Carrère and Hastings); a library wing was considered as an alternative. The former Main Gates face DE 141 (1926–1927, Edward Canby May). A late change to the house was the stained glass window of the stairhall (1933–1934, Massena and du Pont). As early as 1936, Jessie declared her intention of keeping the house intact as a memorial to her husband's taste and high standard of culture. Nemours remained astonishingly unchanged, and following her death, it opened to the public as a museum in 1977.