Mayor Jenny Durkan planned to propose her own accessory dwelling unit (ADU) legislation that would have restricted homeowners’ ability to build second and third units on their property, going far beyond the limitations in the legislation the city council passed unanimously yesterday afternoon.

The restrictions Durkan proposed would have been more lenient than previous regulations, which had resulted in just a handful of ADUs per year, but would have included many provisions requested by ADU opponents, including parking requirements for second ADUs, preserving the current owner occupancy requirement, and imposing new limits  on the size of backyard units.

Ultimately, as I reported this morning (item 2), Durkan did not propose her own legislation, and the bill the council passed yesterday does not include any of these restrictions. Still, Durkan’s ADU proposal gives a glimpse into her thinking about how much the city should limit how many people (and what kind of people) should be allowed to live in single-family neighborhoods.

This report is based on documents I received through a records request filed in March. The mayor’s office provided unredacted versions of these documents this morning.

First, the mayor set out her goals in drafting her own ADU legislation: “1. Encourage ADUs—especially affordable ADUs—throughout Seattle’s single-family neighborhoods. 2. Prevent speculative development and the demolition of existing single-family homes.” Her plan also laid out a set of “principles,” which included “Retain existing single-family neighborhood character.”

To those ends, here’s what the mayor’s proposal (which, again, was never sent to the council as legislation) might have done:

1. Imposed a cap of 1,000 accessory units permitted per year. (The legislation the council passed includes no such restriction.)

2. Required homeowners building a second ADU to sign a legally binding document stating that they would never use that ADU as an Airbnb (a new restriction that would allow someone to own two houses on adjoining lots and rent one as an Airbnb, but would ban a neighbor with two ADUs from renting out their backyard unit).

3. Required two years of continuous ownership before a homeowner could build a second ADU, such as a backyard cottage in a house that already has a basement apartment. This restriction went further than council member Lisa Herbold’s proposal for a one-year ownership requirement, which failed; the legislation the council passed does not include any ownership-related restrictions on ADU construction.

4. Required homeowners to build one off-street parking space when they build a second ADU. Notes from staff on the mayor’s proposal indicate that “many infill parcels, especially those without alley access, cannot easily accommodate off-street parking, making this requirement a significant impediment to ADU development.” The legislation that passed yesterday includes no parking mandate.

5. Imposed a new floor-area ratio (a measure of maximum density) on detached units while eliminating the previous minimum lot size of 5,000 square feet. Although getting rid of maximum lot sizes sounds like a good thing, in practice, this measure would have little practical impact while imposing a new restriction on what people on smaller lots could build. I’ve explained this in a bit more detail below*, but the impact would be that any lot smaller than 5,000 square feet would have to build a backyard unit smaller than 1,000 square feet—and the smaller the lot, the smaller the cottage. In contrast, O’Brien’s legislation allows backyard cottages of up to 1,000 square feet on all lots, subject to the city’s existing maximum lot coverage of 35 percent.

Although getting rid of the minimum lot size entirely might seem preferable, the impact would be tiny—according to the city, just 7 percent of the single-family lots in Seattle are smaller than 3,200 square feet, and ADUs on very small lots are unlikely for the reasons I explain below.

6. Required a homeowner or a homeowner’s family member to live on the property for at least six months out of every year. O’Brien’s legislation got rid of the existing six-month owner occupancy requirement because it effectively banned renters from living in at least one of the units on lots with an ADU (suggesting that backyard-cottage renters require owner supervision.) Durkan’s proposal would have continued to prevent renters from occupying every unit on lots with ADUs, but allowed family members to serve as owner proxies. The proposal doesn’t define “family member,” but other elements of the municipal code limit the number of people who can live on a single lot unless they are “related,” a term that is undefined in the code.

Because I filed my request for these documents in March, they don’t include any discussions that happened after April 1 that might shed light on why Durkan decided not to propose her own ADU legislation. The mayor’s office did not immediately respond to a question about why they dropped the proposal this afternoon.

*Two hypothetical examples illustrate the impact of this change on lots of two different sizes.

A homeowner with a 4,000-square-foot lot could cover a total of 1,400 square feet of that lot with buildings, subject to the maximum height limit of about 30 feet. That could include, say, a 1,600-square-foot two story house (covering 800 square feet of the lot) and a two-story, 1,000-square-foot backyard cottage (covering 500 square feet). Under Durkan’s proposal, though, the backyard cottage would also be restricted by the 0.2 FAR, limiting it to a total of 800 square feet no matter how the rest of the lot is configured. This is the limit that existed before O’Brien’s legislation raised it to 1,000 square feet, so in this case Durkan’s proposal would have preserved the old status quo.

A homeowner with a 2,500-square-foot lot, who couldn’t build a backyard cottage under the rules adopted yesterday, would theoretically be able to do so under Durkan’s proposal. But the restrictions would make this exceedingly unlikely, because the backyard cottage would be limited to a total of 500 square feet—on a lot where only 875 square feet can be developed in the first place. Playing this out presents some very unlikely scenarios, such as a tiny front house towered over by a narrow two-story backyard tower. The point is, the effect of these restrictions would have been primarily to limit the size of backyard units, not to expand homeowners’ ability to build them.