Octagon houses were a unique house style briefly popular in the 1850s in the United States and Canada. They are characterised by an octagonal (eight-sided) plan, and often feature a flat roof and a veranda all round. On the 2645 Cough Street there is the McElroy Octagon House, one of the three remaining octagon houses in San Francisco. The McElroy Octagon House at Cow Hollow is open to the public for tours. On February 23, 1972, it was added to the National Register of Historic Places.