EIS Scoping Report for Backyard Cottages in Seattle Released


On Monday, a scoping report on reforming backyard cottage (also known as “accessory dwelling units” or “ADUs”) regulations in Seattle was released. The scoping report summarizes the feedback that the city received and how the city intends to respond through development of a Draft Environmental Impact Statement (EIS). The Draft EIS will consider a range of alternatives and identify possible mitigation measures to ensure compatibility.

According to the report, over a thousand comments were received during the initial comment period in October. These comments are summarized thematically. The breadth of them is far reaching touching on all elements of environment, such as light, aesthetics, built form, and transportation. Many comments requested that the EIS process consider alternatives well beyond the topic of ADUs.

Teal indicates the general single-family zone areas being evaluated by the EIS process. (City of Seattle)
Teal indicates the general single-family zone areas being evaluated by the EIS process. (City of Seattle)

“Several comments addressed the number and composition of alternatives, including requests that we expand the proposed action alternative or add a third alternative in order to broaden the scope of the analysis,” the report states. “Some comments suggested a third alternative composed of a more aggressive scenario that allows duplexes, triplexes, and small apartments and considers smaller minimum lot sizes for subdivision in single-family zones. Others requested an alternative whose intensity is between our proposed Alternative 1 (No Action) and Alternative 2 by excluding certain changes, such as leaving the owner-occupancy requirement in place, or through other actions that restrict ADU production.”

The EIS will consider three separate alternatives that would affect single-family zones outside of urban villages. Typically, only on action alternative and one no action alternative are required, but the city has chosen to include an additional action alternative in response to range of requests for a wider evaluation of regulatory changes.

The EIS will conduct an evaluation of possible impacts by changes in land use regulations by considering unique situations, such as the difference between areas with large lots and small lots, areas with and without alley access, and areas with and without transit access. The evaluation will also consider impacts from different scales of development, such as at the block level and individual site level.

The action alternatives will consider the following regulatory changes:

  • Varing parking requirements;
  • Allowing two attached ADUs on a lot;
  • Allowing one attached and one detached ADU on a lot;
  • Allowing ADUs to maximum square footage in certain cases;
  • Establishing a floor area ratio limit in single-family zones;
  • Varying owner-occupancy requirements;
  • Allowing one or two additional feet in height for detached ADUs when they meet green roof standards;
  • Requiring participation in the Mandatory Housing Affordability program; and
  • Varying household size standards.

There were some suggestions that have been set aside for the purposes of further evaluation. Major ones that didn’t make the cut include: reducing the minimum lot size for subdivision in single-family zones, rezoning single-family zones (e.g., rezoning to Residential Small Lot, a denser residential zoning type), and further restricting development in single-family zones.

For now, city staff are in the process of preparing the Draft EIS, which should be released later this year for public comment.

City Begins EIS Process on Backyard Cottage Proposal, Comment by November 1

We hope you loved this article. If so, please consider subscribing or donating. The Urbanist is a 501(c)(4) nonprofit that depends on donations from readers like you.

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.

Inline Feedbacks
View all comments

So much ink has been spilled on an issue that really just amounts to trying to make rich white people feel ok about change in their city. How many ADUs/DADUs will actually get built AND actually house people (not someone’s “home office” or “studio space” or whatever most of these things get used for)?

I know they appeal to rich white people. For the record, I AM ONE OF THOSE WHITE PEOPLE. But I’m not deluded into the idea that this is some kind of great solution. It’s a tiny, tiny piece. I’m just trying to be honest about why many people like it. We like it because it feels comfortable to our well-heeled sensibilities. It’s non-disruptive and changes things slowly. That’s about our hangups and our impression of how to protect our investments. It’s not much of an urban planning solution.

Preston Sahabu

It’s the most politically feasible thing we can do to increase density in single-family zones, but that doesn’t mean it strikes the best balance between feasible and effective at increasing density. From most to least feasible in my mind:

– ADUs
– upzone all SF to Residential Small Lot
– expand urban villages
– upzone all SF to Lowrise
– eliminate zoning and institute a land value tax

I’d like to see more investigation into the RSL concept, as it could lead to surprising density (https://www.strongtowns.org/journal/2018/1/3/comparing-approaches-to-achieving-density ) while being palatable to neighborhood advocates.


It could be huge. In Vancouver, 35% of their houses have ADUs. In Seattle, it is 1%. There are 130,000 houses in Seattle, So if we caught up to Vancouver, it would mean about 45,000 new units. That would be a major increase in units that would happen along with the increased number of apartments going in.

Furthermore, building these type of units tend to be very cheap. As a result, they are likely to happen, even if we experience a rental bubble. If you tear down a house and put up an apartment, you throw away the house (which means it only makes sense if you can get a lot for the apartments). Same is true for just about any business, even a parking lot. But with an ADU, you keep the house, and only have to pay for construction. In general, construction like this is quite cheap (as long as the rules allow flexibility). The result could be a lot of cheap units, spread out throughout the city.


Do we see cost controls and greater accessibility in Vancouver as a result of all those ADUs. Superficially, it appears not.