 In 1920, the maximum building height in Chicago was increased to , and unoccupied structures on a building, such as ornamental towers, were allowed to extend up to high.Additional changes came in 1923, with taller occupied towers being permitted for the first time, but subject to controls on overall volumes.The main building of a skyscraper could be up to tall, and a tower could be built on up to 25 percent of the lot, but the tower could not have a volume of more than a sixth of the main block.