The Pentagon

38.8709°N, 77.0547°W.

The Pentagon in Arlington, Virginia, immediately across the Potomac River from Washington DC — headquarters of the United States Department of Defense and the largest office building in the world by floor area, with 6.5 million square feet across 17.5 kilometres of corridors. The structure was designed by George Bergstrom and built in 16 months between September 1941 and January 1943 — an extraordinary construction speed dictated by the entry of the United States into the Second World War in December 1941. The five-sided plan was originally designed to fit a pentagonal tract of land at a different proposed site; when the building was relocated to its current rectangular site after concerns from President Roosevelt about the original location's proximity to Arlington Cemetery, the pentagonal plan was retained because the design work was already complete. The building has five concentric rings around a central 5-acre courtyard, designed so that any two points are connected by a maximum 7-minute walk.

The largest office building in the world, in a five-sided plan retained for the wrong site by accident of design schedule.