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FAQs - Development Services - Building & Planning
Accessory Dwelling Units (ADU's)

Q: Where can the guidelines be found for ADU's?

A: 

The Housing Element of the City of Mercer Islandís Comprehensive Plan states that while density and zoning in single family zones will not change, housing opportunities can be created by allowing accessory dwelling units (ADUs) to be incorporated into single family residences. These ADUs must adhere to specific guidelines that protect the character of the neighborhood contained in MICC 19.02.030. These accessory dwelling units provide housing and have the added benefit of providing homeowners with a means of obtaining rental income, companionship, security and services through tenants in either the accessory dwelling unit or principle unit of the single-family dwelling.

Click here to access highlights of building code regulations specific to ADUs.



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Q: Where are ADUs allowed?

A: One accessory dwelling unit is permitted as subordinate to an existing single family dwelling on any single-family residentially zoned property, provided the primary and accessory units comply with all development code provisions for single-family dwellings and accessory dwelling units contained in MICC Chapter 19.02.


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Q: Can both the primary single family residence and the ADU be rented?

A: No, an owner of the property or an immediate family member of the property owner must occupy either the principal dwelling unit or the accessory dwelling unit. Owner occupancy is defined as a property owner, as reflected in the title records, who makes his or her legal residence at the site, as evidenced by voter registration, vehicle registration, or similar means and actually resides at the site more than six months out of any given year.


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Q: How large can an ADU be?

A: The square footage of the accessory dwelling unit shall be a minimum of 220 square feet and a maximum of 900 square feet, excluding any garage area, provided the square footage of the accessory dwelling unit shall not exceed 80 percent of the total square footage of the primary dwelling unit, excluding the garage area as it exists or as it may be modified.


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Q: Where can an ADU be located on a lot?

A: The accessory dwelling unit may be added to or included within the principal unit, or located in a detached structure. Additions to an existing structure or newly constructed detached structures created for the purpose of developing an accessory dwelling unit shall be designed consistent with the existing roof pitch, siding and the windows of the principal dwelling unit.

The ADU must comply will all development code provisions for single family dwellings including height and setbacks, and the ADU shall be included as part of the impervious surface and floor area limitations for the subject property. The accessory dwelling unit may only be located on lots that are currently conforming to the Cityís development regulations.



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Q: What about parking spaces?

A: All single family dwellings with an accessory dwelling unit must meet the parking requirements applicable to the dwelling if it did not have such an accessory dwelling unit. The Unified Land Development Code requires that each single family dwelling have at least three parking spaces sufficient in size to park a passenger automobile and providing that at least two of the stalls shall be covered stalls.


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Q: Are there any special requirements for where the entrance of the ADU is located?

A: Yes, the single-family dwelling containing the accessory dwelling unit shall have only one entrance on each front or street side of the residence.


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Q: Do I need a building permit for an ADU?

A: Yes, a building permit is required. The accessory dwelling unit shall comply with all standards for health and safety. See "Codes and Design Criteria." (Click here)


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Q: How do I get approval to have an ADU?

A: The property owner must apply for an Accessory Dwelling Unit permit. The application must include an affidavit signed by the property owner affirming that the owner or an immediate family member will occupy the principal dwelling unit or accessory dwelling unit for more than six months per year. The applicant must also record a document with the King County Recorder's Office which runs with the land and identifies the address of the property, states that the owner(s) resides in either the principal dwelling unit or the accessory dwelling unit, includes a statement that the owner(s) will notify any prospective purchasers of the limitations of MICC Section 19.05.030 related to ADUs, and provides for the removal of the accessory dwelling unit if any of the requirement of the Unified Land Development Code are violated.

Once the City receives an application for an Accessory Dwelling Unit, the City processes the request which includes mailing notice of the application to all property owners within 300 feet of the lot where the ADU will be located, giving the public the opportunity to provide feedback and comments. The property owner is required to post a public notice sign for a minimum of 10 days on the property. The public comment period is open for a minimum of 10 days, at the close of the public comment period, staff considers all written comments from the public and ensures that the proposed ADU has met all of the requirements of MICC 19.02.030. Approval or denial of the ADU is an administrative determination and the decision is mailed to the property owner, applicant and all members of the public who provided written comments within the public comment period. The administrative determination can be appealed by any party of record within 14 days of the issuance of the Cityís decision. The appeal will be heard before the Cityís Planning Commission.



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