No person or persons shall excavate or fill or cause to be excavated or filled any portion of a lot or plot of land within 20 feet of any adjoining property or street line so that the slope of the ground within 20 feet of the property or street line is greater than 30° from a horizontal plane when measured from the adjoining property or street line, except when a greater slope is permitted in accordance with §
225-8, Accessory structures or buildings.