This chapter establishes the standards for the location and development of accessory dwelling units in residential zones.  The purpose of accessory dwelling units is to create new housing units that complement the principal dwellings on the properties on which they are located. They can increase the housing stock of existing neighborhoods in a manner that is less intense than alternatives.  Accessory dwelling units allow more efficient use of existing housing stock and infrastructure and provide a mix of housing that responds to changing family needs and smaller households. They provide a means for residents, particularly seniors, single parents, and families with grown children, to remain in their homes and neighborhoods, and obtain extra income, security, companionship and services; and provide a broader range of accessible and more affordable housing.

Date Passed: Monday, June 2, 2025

Effective Date: Friday, July 11, 2025

ORD C36696 Section 3

0.  
1. Where the Regulations Apply.

Attached and detached accessory dwelling units are permitted in the RA through RHD zones, including planned unit developments, subject to the limitations of subsection (B) of this section.

1.  
2. Limitation.

Two accessory dwelling units are allowed per lot in the RA, R1, R2, RMF, and RHD zones subject to the development standards of the underlying zoning district.

2.  
3. ADU versus principal dwelling.

Section 17C.300.130(A)(1) SMC establishes the methods by which an ADU may be created. In cases where a proposed dwelling unit meets the definition and criteria of both an ADU and an additional principal dwelling (e.g., the second unit of a duplex or a second single-unit residential building on a lot), applicants may choose whether the proposed dwelling unit is permitted as an ADU or a principal dwelling.

Date Passed: Monday, June 2, 2025

Effective Date: Friday, July 11, 2025

ORD C36696 Section 3

1. Maximum Size.

1. 0.  
   1. Attached ADU.

Before the establishment of an attached ADU the floor area of the principal structure, excluding an attached garage, must be not less than one thousand (1,000) square feet.

1. 1. 1. The attached ADU must be not more than one thousand (1,000) square feet, excluding any related garage area.

1. 1. 2. The conversion of an existing interior basement or attic space of a principal structure into an ADU may exceed the maximum floor area for an attached ADU specified in subsection (1)(a) of this subsection.

1. 1.  
   2. Detached ADU.

1. 1. 1. The maximum detached ADU size is subject to building coverage per chapter 17C.300.130(B)(3) SMC; and

1. 1. 2. A detached ADU shall not exceed one thousand (1,000) square feet of floor area.

1. 1. 3. The maximum detached ADU size is subject to the maximum building footprint standards for ADUs in Table 17C.111.205-2.

2. Occupancy for Short-Term Rentals.

Where a lot with an ADU also has a Short-Term Rental under chapter 17C.316 SMC, one of the dwelling units on the lot shall be occupied by one or more owners of the property as the owner’s permanent and principal residence. The owner-occupant must occupy the owner-occupied dwelling unit for more than six months of each calendar year. The owner-occupant may not receive rent for the owner-occupied dwelling unit. If a complaint that an owner has violated these requirements is filed, the owner shall:

1. 1. submit evidence to the director showing good cause, such as a job dislocation, sabbatical leave, education or illness, for waiver of this requirement for up to one year absence from the property. Upon such showing the director may waive the requirement;

1. 2. re-occupy the structure; or

1. 3. remove the accessory dwelling unit.

Date Passed: Monday, June 2, 2025

Effective Date: Friday, July 11, 2025

ORD C36696 Section 3

1. Application.

Any property owner seeking to establish an ADU must obtain a building permit and a certificate of occupancy from the building services department.

Date Passed: Monday, June 2, 2025

Effective Date: Friday, July 11, 2025

ORD C36696 Section 3

1. Development Standards – Requirements for All Accessory Dwelling Units.

All accessory dwelling units must meet the following:

1. 0.  
   1. Creation.

1. 1. 0.  
      1. An accessory dwelling unit may only be created through the following methods:

1. 1. 1.  
      2. Converting existing living area, attic, basement or garage.

1. 1. 2.  
      3. Adding floor area.

1. 1. 3.  
      4. Constructing a detached accessory dwelling unit on a site with an existing residential use.

1. 1. 4.  
      5. Constructing a residential use with an attached or detached accessory dwelling unit.

1. 1. 5.  
      6. In the R1, R2, RMF, or RHD zone, constructing an attached or detached accessory dwelling unit on a site with any existing or new principal structure (including non-residential uses or structures). Any structure shall comply with all applicable building, fire, and engineering standards

1. 1.  
   2. Number of Residents.

The total number of individuals that reside in all units on the site may not exceed any lawful limits on occupant load per square foot or generally applicable health and safety provisions as established by applicable building or fire code, as provided in RCW 35.21.682.

1. 2.  
   3. Parking.

Parking requirements are provided in Chapter 17C.230.

2. Additional Development Standards for Detached ADUs.

1. Setbacks.

Except for conversion of existing accessory structures, the accessory dwelling unit must be:

1. 1. 0.  
      1. as specified for setbacks in Table 17C.111.205-2 for accessory dwelling units.

2. Height.

The maximum height allowed for a detached accessory dwelling unit is provided in Table 17C.111.205-2.

3. Bulk Limitation.

1. The maximum building footprint for accessory dwelling units is provided in Table 17C.111.205-2.

2. Attached ADUs shall be included in the maximum building footprint per primary building as provided in Table 17C.111.205-2.

4. Conversion of Existing Detached Accessory Structures.

1. Conversion of an existing detached accessory structure that is in a front building setback required by Table 17C.111.205-2 is allowed as provided. Conversion of an existing detached accessory structure that is in a rear or side building setback is allowed as provided by chapter 17C.111.235 SMC, Setbacks, and chapter 17C.111.240 SMC, Accessory Structures.

2. If the accessory dwelling unit is proposed for an existing detached accessory structure that meets any of the standards of subsections (B)(2) and (3) of this section, alterations that will move the structure out of conformance with the standards that are met are not allowed.

3. If the accessory dwelling unit is proposed as a conversion of an existing detached accessory structure or a portion of the building, and any floor area is added to the existing detached accessory structure to accommodate an ADU, then the entire structure must meet the underlying zoning development standards.

3. Utilities and Addressing.

The ADU must utilize those municipal utilities and address established for the principal dwelling unit.

4. Code Compliance.

The ADU must meet all technical code standards of this title including building, electrical, fire, and plumbing code requirements and permits.

Date Passed: Monday, June 2, 2025

Effective Date: Friday, July 11, 2025

ORD C36696 Section 3

1. Transfer.

In the case where a lot with an accessory dwelling unit also has a Short-Term Rental under chapter 17C.316 SMC, an accessory dwelling unit permit is not transferable to any other property or any other person except to the new owner of the subject property when the property will be owner occupied.

2. Expiration.

Approval of an accessory dwelling unit expires when the:

1. 1. accessory dwelling unit is altered and is thus no longer in conformance with the plans approved by the Development Services Department; or

1. 2. property ceases to maintain the required off-street parking spaces for the accessory and principal dwelling units; or

1. 3. in the case where a lot with an accessory dwelling unit also has a Short-Term Rental under chapter 17C.316 SMC, legal titleholder of the property ceases to own and reside in either the principal or the accessory dwelling unit.

Date Passed: Monday, October 20, 2025

Effective Date: Friday, November 28, 2025

ORD C36753 Section 23