The largest building in the world, according to the Guinness Book of World Records, is Boeing's Everett Plant, located in Everett, Washington, in the US. The famous passenger aircraft, the 747, 767, and 777 are assembled there. When constructed in 1968, the Everett Plant boasted 5.64 million m³ (200 million cu ft) of usable floor space. Since then, it has been expanded twice, 45 percent in 1980 for the 767 assembly line, and another 50 percent in 1993 for 777 assembly. The site's total footprint is 1,025 acres (415 hectares), with 282 acres (113 hectares) of building area, and 215 acres (86 hectares) for paved yards and parking. This makes the world's largest building's area equivalent to a little over a square kilometer.
Many of the world's largest buildings have to do with the assembly of aircraft or spacecraft. For example, the Aerium, in Brandenburg, Germany, about half the size of the Everett plant, was originally constructed as the assembly area for a giant airship which was never built. The NASA Vehicle Assembly Building, in Florida, is about a third the size of the Everett Plant, as was built to assemble the space shuttles.
There are several buildings a bit larger than the Everett Plant, but generally are not given credit because not all their floor space is usable. For example, the Aalsmeer Flower Auction, Aalsmeer, the Netherlands, is a bit larger, but not all its floorspace is usable, ranking it lower than the Everett Plant. It is still qualified as the world's largest building for commercial (as opposed to industrial) use, and by a long shot the world's largest building devoted to flower auctions.
The Great Wall of China and China's Three Gorges Dam are two other structures that challenge the Everett Plant for the world's largest building status. The Great Wall of China is over 6,500 km long, but doesn't qualify as a building because it has no interior and little real usable space. Similar, the Three Gorges Dam does not qualify as a building per se because the vast majority of its interior is not usable in the fashion that is usually meant by "building."