Grey Towers in Glenside, PA was constructed in 1893 as the estate of sugar magnate William Welsh Harrison. In 1929, two years after Harrison's death, Arcadia University purchased all 53,564 square feet of it for $712,500 and it’s now a multi-use building on campus with select freshmen allowed to live inside.

#21 Grey Towers Castle

With 132 rooms, including 35 bathrooms, and 55,000 square feet, the White House in Washington, DC has been home to every sitting president since James Monroe in 1817. Also known as the "President's Palace," "President's House" and the "Executive Mansion," President Theodore Roosevelt gave the mansion its current name in 1901.

#20 White House

Entrepreneur William Chapman Ralston purchased a two-story Belmont, California villa from an Italian nobleman in 1864 and in four years, expanded it to a whopping 55,000 square feet. In 1922, the Sisters of Notre Dame de Namur purchased it to use as a classroom hall for their university students and teachers, but since 2012, the mansion has been under repairs.

#19 Ralston Hall

American industrialist Andrew Carnegie forever altered New York’s Fifth Avenue when he built his 64-room, 56,368 square foot mansion in 1902. Sophisticated in its central heating, cooling technology and modern steel-frame construction, the mansion opened in 1976 as the Smithsonian Institution's Cooper-Hewitt Museum.

#18 Carnegie Mansion

Whitehall was built in 1902 in Palm Beach for railroad tycoon Henry Flagler, who was widely regarded as the man responsible for turning Florida into a tourist destination. Flagler died in 1913 after suffering a fall in Whitehall but today, the 55-room, 60,000-square-foot mansion is open to visitors and still drips with marble, gilding, and murals.

#17 Whitehall