Residential accessory use or structure which is subordinate and incidental to a permitted residential dwelling unit. Residential accessory uses and structures may include, but are not limited to, garage, carport, storage shed, noncommercial greenhouse, fence six feet or less in height and the accessory uses listed above. Accessory structures constructed after the effective date of the ordinance codified in this title shall be less than or equal to 65 percent of gross floor area of the primary residence or 1,200 square feet, whichever is greater, and less than or equal to 20 feet in height;