Hearst Castle, California

Hearst Castle, built by architect Julia Morgan between 1919 and 1947, was the largest private home in the U.S. during its construction, with a 90,080-square-foot main house featuring 38 bedrooms and 41 bathrooms. Its European art and antiquities collection is valued at over $300 million, and the estate's Neptune Pool holds 345,000 gallons of heated water. It is 40% larger than #5 Fallingwater's total footprint, though its construction cost of $10 million in 1947 dollars is dwarfed by modern equivalents. Now a California State Historic Park, it attracts 750,000 annual visitors, making it 30% more visited than the average historic house. Annual operating expenses exceed $4 million.