The planning commission may modify the standards and requirements of this ordinance if the subdivision is located on land of 20 percent or greater slope. To minimize disturbance of the existing grade and to take advantage of natural building sites, modification may concern alignment, width and improvement of streets, and building site locations. If modification involves the creation of some lots of less than the minimum area, the average area of lots in the subdivision shall equal the density established for the area under the zoning in effect.