The largest building in the world is about to complete, which is based in China.

Local authorities in China said that the 100-metre-high New Century Global Centre is a symbol of the spread of China's boom, which is 500 meters long and 400 meters wide, with 1.7 million square meters of floor space, big enough to hold 20 Sydney Opera Houses.

By comparison, the Pentagon in Washington, still one of the world's largest office buildings, is barely a third of the size with a mere 600,000 square meters of floor space, the Sydney Morning Herald reports.