Accessory Dwelling Units

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What is an Accessory Dwelling Unit?

An accessory dwelling unit, or ADU, means a second living unit incorporated within an existing single-family home. The owner of the primary residence owns the ADU. Accessory dwelling units are considered a housing option for senior citizens, students, young couples, relatives of property owners, and other persons needing convenient and more affordable housing—yet must be compatible with existing family neighborhoods.

Are ADUs Permitted?

ADUs are permitted if they are subordinate to single-family residences, subject to the following:

  • Owner occupancy. An owner of the property must occupy either the primary residence or the ADU.
  • Number of occupants. The combined number of occupants in the primary residence and the ADU may not exceed the number allowed by the definition of “family,” Land Use Code (LUC) 20.50.020
  • Size of ADU. The ADU must contain at least 300 square feet, and no more than 800 square feet, excluding garage area. If the ADU is located on a single floor, the Development Services Director may allow an increase so that floor area may be efficiently used, provided all other standards for ADUs are met.
  • Size in relation to house. The area of the ADU may not exceed 40% of the combined area of the ADU and primary residence. Garage space is not included in this calculation.
  • Existing single-family dwellings. ADUs are only allowed in existing single-family dwellings that received their final inspection approval at three years prior.
  • Parking. The ADU must have at least one off-street parking space in addition to any parking required for the primary residence.
  • ADU entry. Any new entry door necessary to serve the ADU may be located as follows:
  • On the rear of the house.
  • On a side of the house, but not facing the street.
  • On the front of the house, but only if no other entry door on the front of the house exists.
  • If the house already has two entry doors facing the street, one of those doors may be used to provide access to the ADU.
  • 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.

Visit our Accessory Dwelling Unit Permit page to learn about permit requirements and how to apply.

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.

Can an ADU count as affordable housing?

No. An ADU may not be used to satisfy affordable housing requirements of LUC 20.20.128.

What does registration of an ADU involve?

First, before the ADU is approved by the city, the property owner must submit an ADU registration application form with the following information:

  • A statement that at least one property owner will occupy either the primary residence or the ADU, and
  • A statement agreeing to the limits on total number of residents and other standards summarized above.

Second, Development Services will inspect the site to confirm that size limits, entrance locations, and technical code requirements are met and that required parking is provided.

Third, the applicant records the registration form as a deed restriction with the King County Department of Records and Elections. The owner pays the recording fee.

Fourth, Development Services will provide notice of the registration of the ADU to owners of property within 500 feet of the site.

How can ADU registration be canceled?

The property owner may file a certificate with Development Services for recording with the King County Department of Records and Elections. Cancellation may also result from an enforcement action. The cancellation certificate will confirm that the residence has reverted back to use as a single-family dwelling.

Where can I get additional information?