The original plans for the Tesla Gigafactory call for a facility with a footprint of 5.8 million square feet, on two stories for 10 million square feet of floor space. That would already give it the largest footprint of any building in the world. But that could just end up proving the starting point.
The Gigafactory is being built in a modular fashion, and Tesla is reportedly buying up adjacent plots of land in order to expand the facility. Another 1,200 acres have reportedly already been acquired, with an additional 350 or more on top of that also being looked into. Three modular blocks in addition to the original four could take it up to 24 million square feet of floor space. That would not only give it the biggest footprint, but also the most floorspace of any building in the world – a boast currently claimed by the New Century Global Center, a multi-use complex in Chengdu, China, that incorporates a offices, hotels, movie theaters, a university campus, a skating rink, and a water park on its 18+ million square feet of space.
It remains to be seen whether the interior volume of the Gigafactory would end up displacing the Boeing factory in Everett, WA. By our calculations, though, if the Gigafactory is built with ceilings any higher than 20 feet, it would take that record, too. It'll likely emerge as the largest automotive assembly plant as well, but it won't be used exclusively for making vehicles. Much of it would be dedicated to manufacturing batteries for use both electric cars and real estate (residential, commercial, and industrial) applications.