The Snohomish County Council has liberalized accessory dwelling unit (ADU) regulations once again in urban unincorporated areas, building on reforms enacted in 2021. The county council moved to provide more flexibility in the mixture of ADUs allowed on lots, allow ADUs to be created with more housing types, and revise and eliminate some performance standards in the latest package of reforms.
In 2021, the county council made sweeping changes to encourage ADUs in urban areas. Snohomish County includes several unincorporated communities that are relatively dense and fast-growing, including areas around future light rail stations planned at Mariner and Ash Way Park and Ride. Overall, 376,700 residents live in unincorporated Snohomish County — 43% of the county’s population.
Ahead of the county council’s unanimous vote earlier this month, Councilmember Megan Dunn, who was a sponsor of the legislation last year, expressed her support for the ordinance. “I just want to appreciate all the staff and all the input and working on this for a number of years, actually,” Dunn said. “We’re excited for this, for improving home ownership, affordability, and accessibility.”
Part of the impetus for the county’s ADU reform are new state mandates to reduce barriers to ADUs and encourage their creation, which are due on June 30. The scope of the state mandates is broad, touching on the quantity, type, and configuration of ADUs that must be allowed, as well as size and bulk limitations on ADUs.
Under previous regulations, Snohomish County only permitted ADUs when proposed on a lot with a single-family home. Up to two ADUs could be formed on such lots, provided that one is an attached accessory dwelling unit (AADU) and one is a detached accessory dwelling unit (DADU). Two of the same type — two AADUs or two DADUs — were not permitted under the 2021 rules.
The new regulations will provide more flexibility by allowing ADUs on lots with single-family homes, regardless whether they are in a detached or attached form, and on lots with duplex homes. These housing types are allowed in all low- to high-density urban residential zones (R-9,600, R-8,400, R-7,200, T, LDMR, and MR). The configuration and quantity of ADUs that will be allowed for these housing types differs as follows:
| Principal Unit Type | ADU Allowance |
|---|---|
| Single-family detached home | A maximum of two ADUs in the following configurations: – One AADU and one DADU; – Two AADUs; or – Two DADUs (in two structures or attached in one structure). |
| Single-family attached home | A maximum of one ADU in the following configurations: – One AADU; – One DADU; or – Two DADUs in one structure if the structure straddles adjacent single-family attached homes and each DADU sits on the respective lot. |
| Duplex | A maximum of two ADUs on the duplex lot in the following configurations: – One AADU and one DADU; – Two AADUs; or – Two DADUs (in two structures or attached in one structure). |
The new legislation modifies bulk regulations to support the creation of ADUs. This includes increased lot coverage to a maximum of 55%, reduced setbacks from private roads to five feet for principal units and ADUs, and reduced setbacks from most public roads to five feet for ADUs. As a further incentive for ADUs, the regulations grant land use administrators leeway to permit conversion of existing nonconforming structures into ADUs even when doing so would contravene setback and lot coverage standards.
Architectural standards that had required similar aesthetics and design to a principal unit will no longer apply. That removes a small but discretionary barrier for developers and homeowners to design ADUs as they wish, offering more creativity, flexibility, and certainty.
While the county eliminated owner-occupancy requirements for ADUs or their related principal units in 2021, the ADU definition required the ADUs be under the same ownership as their related principal unit. The adopted ordinance strikes this text, allowing for ADUs to be converted to condominiums and sold separately.
ADUs will still be subject to other existing development regulations, however. Key examples include:
- A size limitation of 1,200 square feet of floor area (not inclusive of garages, porches, unfinished basements, and unheated storage areas) for each ADU;
- Setbacks of at least 5 feet from side and rear lot lines, and potentially greater depending upon the zoning circumstances;
- A height limit of at least 30 feet, and potentially greater depending upon the zone;
- No off-street parking requirements; and
- Documentation that there is sufficient legal water availability and flow and sewage or septic system capacity.
Natalie Reber, a government affairs manager for the Master Builders Association of King and Snohomish Counties, lauded the ADU reforms ahead of the county council’s final adoption of the ordinance.
“These proposed changes align with the Growth Management Act. They support housing affordability and promote sustainable land use policies that benefit all of the residents of Snohomish County,” Reber said. “So we think this is an important first step towards making middle housing a reality in Snohomish County, and we’re very supportive of the ordinance.”
Stephen is a professional urban planner in Puget Sound with a passion for sustainable, livable, and diverse cities. He is especially interested in how policies, regulations, and programs can promote positive outcomes for communities. With stints in great cities like Bellingham and Cork, Stephen currently lives in Seattle. He primarily covers land use and transportation issues and has been with The Urbanist since 2014.