 Typically a building could only rise up from the sidewalk vertically by around , after which the building had to be set back at a set angle to allow sunlight to reach the street below.On a quarter of the site, however, the owner could build as high as they wished, without any further restrictions.A real estate boom occurred in the U.S. after the end of the First World War, with a particular surge in the construction of new skyscrapers between 1925 and 1931.