Snohomish County Liberalizes Backyard Cottage and Basement Apartment Rules

Backyard cottage with woman in patio and sunset.
The pre-approved backyard cottage from Artisan Group on the City of Seattle's ADUniverse website.

Snohomish County’s long-awaited reform of accessory dwelling units (ADUs) has finally come to pass. Yesterday, the Snohomish County Council voted unanimously to pass a package of ADU regulatory reforms for both urban and non-urban unincorporated areas of the county where some 369,000 residents live — two-thirds of whom are in urban areas. Key highlights among the reforms are two ADUs per urban lot, elimination of owner-occupancy requirements, and no parking requirements for ADUs in urban areas. The package of regulatory reforms were much broader.

The key adopted changes in the county’s ADU regulations (note that “AADU” means “attached ADU” and “DADU” means “detached ADU”) are as follows:

Snohomish CountyCurrent RegulationAdopted Changes
Number of ADUsOne per lot with a single-family residence, provided that an ADU is not permitted on a lot that has a temporary dwelling unitIn urban zones, one AADU and one DADU are permitted per lot with a single-family residence; and in rural, resource, or other zones, one ADU is permitted per lot with a single-family residence, provided that DADUs are not permitted on lots that do not meet the minimum lot size for the zone and DADUs as mobile homes are only permitted on lots that are 10 acres in size or more
AADU Maximum Size20% to 40% of the floor area of the primary residence, depending up size, up to 1,500 square, provided that an AADU cannot be less than 360 square feet and the primary residence cannot be reduced to less than 900 square feet1,200 square feet, excluding garages, porches, and unfinished basements
DADU Maximum Size40% of the floor area of the primary residence or 850 square feet, whichever is less, provided that a DADU cannot be less than 360 square feet1,200 square feet, excluding garages, porches, and unfinished basements
DADU LocationIn residential, multiple family, and commercial zones, a DADU cannot be located beyond the primary residence front unless well landscaped or compatibleNo longer applicable
ADU Parking One off-street parking space is requiredIn urban zones, no off-street parking space is required; one off-street parking space is required per ADU in other zones
Owner OccupancyAn owner-occupancy covenant must be recordedNo longer applicable
Non-Urban StandardsNot applicableMaximum separation between the primary residence and DADU is generally 100 feet, except for DADUs in a legally constructed accessory structure prior to the ADU reform legislation adoption, and ADUs must use the same driveway as the primary residence
ScreeningADUs must be screened with a six-foot high fence or five-foot wide high intensity landscape screeningNo longer applicable
Substandard LotsADUs are prohibitedNo longer applicable, except that DADUs are not permitted on lots in non-urban zones that do not meet the applicable minimum lot area
Use AuthorizationIn zones where permitted, an administrative conditional use permit is requiredIn zones where permitted, the use is permitted outright

Snohomish County did miss out on other reforms that could have gone further to encourage ADUs in urban areas, such as allowing two AADUs on single-family lots, increasing lot coverage, reducing setbacks, and allowing ADUs for duplexes and townhouses. The county also did not address genuine issues of ADUs being used in rural areas to create massive garages and avoid procedural and regulatory hurdles.

“If we’re looking at affordable housing, green housing, and increasing density, I think it’s critical that we can remove the restriction for the parking spots.”

Snohomish County Councilmember Megan Dunn, who represents District 2 (Mukilteo, Everett, and Tulalip).
Zoning map of Snohomish County. White areas are not under county zoning authority. Non-green areas are generally urban zones and greenish areas are general rural, resource, and farmland zoning. (Snohomish County)
Zoning map of Snohomish County. White areas are not under county zoning authority. Non-green areas are generally urban zones and greenish areas are general rural, resource, and farmland zoning. (Snohomish County)

During the meeting, there were four amendments to the base legislation that was considered with three of them passing. The county council passed back an amendment to the planning commission that would have allowed the creation of ADUs on smaller lots in rural, resource, and other non-urban zones that do not meet the applicable minimum lot size.

The first amendment that did pass addressed the maximum size of ADUs. This set the universal limit on ADUs at 1,200 square feet, not including other features likes basements and attached garages. It closely mimicked the original staff recommendation to the planning commission last year that had proposed limiting ADUs to 1,000 square feet in urban zones and 1,200 square feet in rural, resources, and other zones. The planning commission, however, pushed for a much higher number at 1,600 square feet, which is on the scale of many single-family homes. But the blended, single limit that the county council settled on brings the standard back to earth and better delineates it from a detached single-family home while promoting it as a more affordable alternative.

A second amendment passed that passed provides more flexibility in the location of DADUs on lots outside of urban areas due to site constraints like the location of existing environmental features and septic systems. It allows DADUs to be located more than 100 feet away from the principal single-family dwelling unit if the constrained site criteria are met.

The third amendment that passed eliminated parking requirements for all ADUs in urban areas. Councilmember Megan Dunn championed this amendment and argued for it during the meeting.

County Councilmember Megan Dunn. (Snohomish County)

“Based upon research, occupants of ADUs typically own fewer cars than the average household and mandating parking will undermine efforts to boost construction of green, affordable ADU homes,” Dunn said. “Also, parking quotas thwart one of the cheapest forms of ADUs, which is garage conversions and this will also lower the cost because ADU parking spaces are estimated anywhere from $4,000 and $30,000 per spot. I think if we’re looking at affordable housing, green housing, and increasing density, I think it’s critical that we can remove the restriction for the parking spots.”

Council Chair Stephanie Wright disagreed with the policy proposal. She explained her position on why ADUs should have parking requirements claiming that the residents of ADUs “do have less cars, but they don’t seem to have no cars.”

In her full justification for voting against that amendment, Council Chair Wright said: “This [legislation] would require one car and what we’ve seen in [the urban] area is that even though people are using [public] transportation, tend to still have one car and actually a bigger need for parking because they are taking the bus and doing other activities, but like true Washingtonians we like to have our car for weekend recreation and our trips to Costco and other things.” She also added that she’s received many parking complaints in her urban district and was hesitant to support a parking reduction because there hadn’t been a parking analysis conducted.

Despite her disagreement with the policy, the amendment passed 4-1 and Council Chair Wright supported the overall legislation giving the county council an unanimous decision in favor of ADU reform.

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Stephen is an urban planner 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. Stephen lives in Kenmore and primarily covers land use and transportation issues for The Urbanist.

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When will the new regulations be adopted?

A Joy

Long awaited by whom? Not by developers or people actually interested in increasing housing. Seattle has relaxed their ADU/DADU regulations twice, promising a new boom in them that never panned out either time. This type of mythical missing middle housing simply has no demand, so you could eliminate all regulation on them and they still wouldn’t amount to a drop in the bucket of desperately needed housing in the region.


Not a “boom” like in LA, but a robust increase in pace in Seattle (which is impressive in the context Doug’s previous piece about the general difficulty of permitting in Seattle)

And drops in the bucket can still scale is there are lots of drops.

A Joy

Ha. That is a joke, right? From 1994 to March 2009, 921 ADU/DADU permits were issued. Another roughly 300 were issued between then and 2016. We’re talking about less than 2,000 units built since the legislation was enacted. The reform in 2006 was supposed to bring in thousands of units a year. A year. And you’re praising hundreds of units in a year well over a decade later? No, this type of building does not scale. Not even a little. With 11,000 people living on the streets right now, 500 units a year won’t even keep up with the homeless ROI.


The relevant rule change for Seattle in that Sightline piece was in 2019, not sure why you are looking at 2006?

Given California completed 16,000 ADUs in just 2019 alone, yes I think greater Seattle can deliver thousands of additional housing units over the next decade under these more liberal regulations if enough cities in the metro adopt them.

A Joy

I’ve been following the fallacy of ADU/DADU construction for much longer than just 2019. I remember the promises from the previous regulations revision, and how little was actually built despite the promises of the time. And I still don’t know why you keep trying to push the relevance of San Francisco statistics on Seattle. They’re not even in the same state, after all. There’s no reason to believe they bear any relevance to our situation.

Stephen Fesler

This code amendment was several years in the works and long-awaited by property owners and developers alike. The data that AJ is pointing to is very instructive. You may believe that ADUs are meaningless to missing middle housing and housing production, and you’d be right that ADUs don’t offer a whole lot in localities that have high barriers to them, rendering them impractical in most applications. But that’s not so in jurisdictions that have ADU regulations using best practices. The changes that Snohomish County adopted have a very good chance of finally producing ADUs in urban areas. You can only be speaking for yourself when saying “people actually interested in increasing housing” because people who actually are interested in that know that ADUs have a role in delivering additional housing opportunities and production output. No one’s claiming that ADUs alone will meet housing targets, but it’s the compounding actions to reform zoning that together help deliver more than if the status quo remains unchanged.


“Should we bet on ADUs & DADUs, ‘plexes, small efficiency dwelling units, stacked flats, ‘plexes to increase housing affordability?”

Yes, all of the above, everywhere. 🙂

A Joy

No, the statistics support the position that nobody actually interested in increasing housing cares about ADU/DADUs, or for that matter the entire mythical “missing middle”. There’s simply no demand for it, and that’s an indisputable fact. The supply is low because the demand is equally low. If missing middle housing had a demand for it, it would have been built and these regulations changed before the turn of the century. Yet they weren’t. Not even in Seattle.

It’s time we started facing reality, not chasing pie in the sky pipe dreams like rowhouses, duplex/triplexes, and ADU/DADUs. Times are desperate and crucial. We can’t afford to screw up this badly in today’s housing shortage. Grown ups need to start making grown up plans.

Douglas Trumm
A Joy

You would be correct. I advocate doing away with “urban villages”, growing the area roughly from SODO to the UW into a unified, dense, larger urban core, and methodically pushing all nonhistoric SFH to the very fringes of the city limits. Sadly Queen Anne, parts of Alki, and a few other areas will likely avoid this integration under historic preservation statutes.

I would also have members of those communities getting together to discuss what the feel of their neighborhood is and how to preserve that through city parks, existing plazas, and street view building aesthetics. But I would also be cautious about letting this process feed NIMBYs.