An accessory dwelling unit is a second living unit incorporated within a single-family home. ADUs are considered a housing option for senior citizens, students, young couples, relatives of property owners and other persons needing convenient and more affordable housing. They must be compatible with existing family neighborhoods.
ADUs are permitted if they are subordinate to single-family residences, subject to the following:
- The size of the ADU is limited to the larger of either:
- 1,200 sq. ft., or
- 40% of the combined area of the ADU and the primary residence, excluding any garage area.
- Parking is required based on the size of the ADU and your property:
- No parking requirement for ADUs less than 1,000 sq. ft. or if the lot is within 1/2 mile of frequent transit.
- Frequent transit includes light rail stations, bus rapid transit stops, or bus stops with service every 15 minutes for 12 hours a day.
- 1 parking space is required for an ADU 1,000 sq. ft. or larger (unless within 1/2 mile of frequent transit).
- For ADUs larger than 2,000 sq. ft.:
- 1 parking space is required if the lot is less than 6,000 sq. ft. in area.
- 2 parking spaces are required if the lot is larger than 6,000 sq. ft. in area.
- No parking requirement for ADUs less than 1,000 sq. ft. or if the lot is within 1/2 mile of frequent transit.
- Compliance with codes. The ADU must satisfy the requirements of all construction codes adopted by the city (Bellevue City Code, Title 23), including building, electrical, fire, and plumbing code requirements. See below for more information.
For more information, see Chapter 20.20.120 of the Land Use Code.
Frequently Asked Questions
What permits do I need to build an ADU?
Only building permits are required to build an ADU, and the type of permit depends on the scope of work:
- Large Addition: Construction of an addition greater than 3,000 gross square feet of new and/or replaced floor area. (BS)
- Addition - less than 3,000 square feet: The addition of less than 3,000 square feet of attached floor space to an existing dwelling. (BR)
- Remodel with Structural Work: Any change within an existing dwelling that does not increase the area and rood sheathing that is being replaced or altered. (BU)
When applying for your permits, you will be asked to indicate whether it is for an ADU, as well as details about the proposed ADU and the property.
If no construction work is required for the ADU, contact Land Use to determine whether the proposed ADU meets all applicable requirements.
Can an ADU be a separate building?
No. The ADU is not permitted in any structure detached from the primary residence. It must be incorporated in the main residence.
Can a home contain an ADU and a home-based business?
No. A property may not contain both an ADU and a home-based business which requires a Home Occupation Permit under LUC 20.30N.
As a result, ADUs cannot be used for short-term rentals, such as Bed and Breakfasts, which require a Home Occupation Permit. ADUs may only be rented for 30-days terms or longer.
Can an ADU count as affordable housing?
No. An ADU may not be used to satisfy affordable housing requirements of LUC 20.20.128.