The outside perimeter of a structure, excluding eave overhangs and other cantilevered elements of the building, that projects no more than 18 inches is referred to as its footprint. The footprint of a building is the area of ground that it covers; thus, the term "footprint of the building" is commonly used to describe the area of land that it occupies. The footprint of a building is important because it determines how much land will be needed for its construction. Footprint figures are usually specified by the zoning ordinance or site plan review committee when reviewing site development plans. If the building height exceeds 18 inches, then an additional 12 inches must be added for clearance. So, a 20-foot-high building would require an area of 40 feet by 60 feet for its construction. Clearance margins are added to the total building area to provide space for doors, windows, and other openings in the exterior walls. These areas are not part of the footprint but are included in the total building area for purposes of site planning. Doors and windows that open onto a public street or sidewalk also increase the risk of intrusion into people's homes. They should be placed so as not to obstruct vehicle traffic or create a security risk for residents.