4-2-110C DEVELOPMENT STANDARDS FOR ACCESSORY DWELLING UNITS1
1. Purpose and Intent: The provisions of this subsection are available as an opportunity to augment local housing inventory, diversify housing type and size, and create opportunities for homeownership. Accessory dwelling units (ADUs) are well-suited for smaller households, older adults aging in place, people with disabilities, individuals on fixed incomes, and others seeking affordable living arrangements.
2. APPLICABILITY: The provisions of this subsection shall apply to the development of ADUs when located on the same lot as a principal dwelling within the following residential zones: Resource Conservation (RC), Residential-1 (R-1), Residential-4 (R-4), Residential-6 (R-6), Residential-8 (R-8), Residential-10 (R-10), and Residential-14 (R-14). ADUs proposed as an accessory use to a religious institution or social service organization shall follow the provisions of RMC 4-2-080A7, Conditions Associated with Zoning Use Tables and RMC 4-9-030D and 4-9-030I, Conditional Use Permits.
3. Additional Requirements: All applicable health and safety standards shall apply. Nothing in this subsection requires the issuance of a building permit for ADUs if other federal, state, and local requirements for a building permit are not met, including but not limited to building, fire, and energy code requirements, adequate access to utility services, emergency services, etc.
4. Development Standards: The following development standards are established for ADUs:
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GENERAL REQUIREMENTS4, 10 |
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Maximum Number of Units24 |
Two (2) ADUs are allowed per legal lot as an accessory use to a principal dwelling. |
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Configuration |
Either one (1) attached ADU and one (1) detached ADU, two (2) attached ADUs, or two (2) detached ADUs. ADUs may be established within or as an addition to the principal dwelling. In RC, R-1, R-4, R-6, and R-8 zones, when two (2) ADUs are proposed, the second ADU shall comply with the location and setback requirements applicable to the principal dwelling. |
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Maximum Gross Floor Area40 |
1,000 sq. ft. |
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Maximum Building Area26, 41 |
1,000 sq. ft. |
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Conversions25 |
If a conversion unit occupies an entire single floor of the primary dwelling, it may be allowed to increase the maximum unit size to efficiently use all the floor area. Conversion units greater than 1,250 sq. ft. shall be classified as an attached dwelling rather than ADU. |
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Parking20 |
A minimum of 1.0 parking space per dwelling unit. No off-street parking required for ADUs located on lots within a one-half mile walking distance of a Major Transit Stop (Area A). |
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Design Standards |
See RMC 4-2-115, Residential Design and Open Space Standards. |
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ATTACHED AND INTERNAL ADUs |
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Setbacks and Building Height |
ADUs shall comply with the standards applicable to primary structures, pursuant to RMC 4-2-110A, Development Standards for Residential Low Density and Medium Density Zones – Primary Structures, and RMC 4-2-110B, Development Standards for Residential High Density Zones – Primary Structures. |
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DETACHED ADUs |
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Front Yard and Secondary Front Yard Setbacks |
ADUs shall comply with the location and front yard setback requirements of the underlying zone, pursuant to RMC 4-2-110A, Development Standards for Residential Low Density and Medium Density Zones – Primary Structures and RMC 4-2-110B, Development Standards for Residential High Density Zones – Primary Structures. Detached ADUs shall be located at least 4 ft. from any residential structure otherwise the structure shall be considered an attached ADU. |
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Side Yard Setbacks |
RC and R-1 zones: 25 ft. R-4, R-6, and R-8 zones: 5 ft. R-10 and R-14 zones: 4 ft. |
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Rear Yard Setbacks |
5 ft. When located within 10 ft. of the rear property line, at least 25% of the lineal length of the rear yard shall remain unoccupied from accessory dwellings. ADUs may be sited at the rear lot line where an alley is present. |
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Building Height23, 42 |
24 ft. measured to the highest point of a flat roof or the highest ridge of a pitched roof. |
(Amd. Ord. 4963, 5-13-2002; Ord. 4999, 1-13-2003; Ord. 5100, 11-1-2004; Ord. 5132, 4-4-2005; Ord. 5450, 3-2-2009; Ord. 5473, 7-13-2009; Ord. 5518, 12-14-2009; Ord. 5590, 2-28-2011; Ord. 5675, 12-3-2012; Ord. 5726, 10-20-2014; Ord. 5744, 1-12-2015; Ord. 5759, 6-22-2015; Ord. 5790, 4-25-2016; Ord. 5841, 6-12-2017; Ord. 5917, 12-10-2018; Ord. 5960 (Att. C), 12-9-2019; Ord. 6002, 12-14-2020; Ord. 6091, 11-28-2022; Ord. 6128 (Att. D), 12-11-2023; Ord. 6160 (Exh. B-2), 6-16-2025; Ord. 6173, 11-17-2025)
Please see Section 4-2-110I, Conditions Associated With Residential Development Standards Tables, for explanation of table footnotes.