FAQs - Development Services - Building & Planning
Garages & Other Accessory Bldgs.
Q: What is an accessory building of a Single Family Residence?
A: An accessory building is a separate building or a portion of the main building, the use of which is related to and supports that of the single-family residence on the same lot. (See ULDC 19.02.040)
Q: Does an accessory building have to meet setback requirements?
A: Yes, with a few exceptions, no accessory buildings, including garages are allowed in any setback.
Q: Does an accessory building have to meet height requirements?
A: Yes, the height of attached accessory buildings is integrated with the calculation of primary structure, in relation to Average Building Elevation and maximum building height. A detached accessory building, a building that does not share a portion of any of its walls with the main building and is separated from the main building by more than five feet, must comply with maximum height requirements but its average building elevation and maximum building height is calculated separately from the primary structure.
Q: How can my neighbor have a small garden shed in his rear yard?
Detached Accessory Building. Accessory buildings are not allowed in required yard setbacks; provided, one detached accessory building with a gross floor area of 200 square feet or less and a height of 12 feet or less may be erected in the rear yard setback. If such an accessory building is to be located less than five feet from any property line, a joint agreement with the adjoining property owner(s) must be executed and recorded with the King County Department of Records and thereafter filed with the city.
Q: Why when I drive around the Island does it appear that some garages are located in the front yard setback?
A: In some instances, garages may be built to within 10 feet of the front property line if the front yard of the lot, measured at the midpoint of the wall of the garage closest to the front yard property line, is more than four feet above or below the existing grade at the point on the front property line closest to the midpoint of the wall of the garage at its proposed location. The height of such garage shall not exceed 12 feet from existing grade for that portion built within the front yard setback.
Q: What if I want to connect my detached garage to my house with a covered walkway?
A: Enclosed or covered pedestrian walkways may be used to connect the main building to a garage or carport. Enclosed pedestrian walkways may not exceed six feet in width and twelve feet in height calculated from the finished grade or thirty feet above average building elevation, whichever is less.
Q: What if I want my enclosed walkway to be elevated and it is higher than 12 feet from finished grade?
A: A pedestrian walkway may be elevated or higher than 12 feet from finished grade; however in doing so the garage and carport regardless of the distance separating the garage or carport from the primary structure, is considered an attached accessory building and the Average Building Elevation and maximum building height of the primary building and the garage/carport are integrated and not permitted to be calculated separately.
Q: What else should I know?
A: This information is summarized from Title 19 of the Mercer Island Municipal Code, "Unified Land Development Code” and is provided for quick reference only. To determine exact requirements, and for other development regulations affecting a parcel or lot, Title 19 should be consulted. The Mercer Island regulations on garages and other accessory buildings are contained in ULDC 19.02.050.