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Seattle evaluates options to encourage backyard cottages, in-law apartments

The City of Seattle continues to seek public input on the environmental review of options to encourage the development of more in-law apartments and backyard cottages that would allow for more rental housing in the city’s single-family zones.

The review evaluates a proposal advanced by Councilmember Mike O’Brien that aims to remove regulatory barriers and make it easier for property owners to build Accessory Dwelling Units (ADUs).

Current City ordinances allow two types of ADUs: in-law apartments inside a single-family home and detached backyard cottages. Seattle homeowners have created 1,591 in-law apartments and 579 backyard cottages since first allowed in 1994 and 2010, respectively. Homeowners received permits to build 263 ADUs last year.

The potential policy changes analyzed through the environmental review process would increase rental housing options in areas where housing is unaffordable to most households. Since 2016, Seattle has been the city with the fastest growing single-family home prices in the nation, with the cost of the average single-family home now $820,000.

Under Seattle’s current zoning, approximately 7400 acres allow for multifamily, commercial, and mixed-use buildings, while 21,000 acres are zoned exclusively for single-family homes. There are roughly 135,000 lots in Seattle’s single-family zones.

Citywide, the rapid pace of construction in 2017 added 8,730 net new homes, most of which were apartments and townhomes. Last year, 88 percent of all new homes were built in Seattle’s urban villages. The number of detached single-family homes in Seattle remained essentially unchanged.

Three options studied

Under current regulations governing ADUs, the homeowner must live on the property, only one ADU is allowed per lot, and the property must include two parking spaces, one for the main home, and one for the ADU.

The City’s Draft Environmental Impact Statement (DEIS) released on May 10th studies three alternatives for Land Use Code changes related to ADUs:

  • Alternative 1: no action; current rules maintained.
  • Alternative 2: one in-law apartment and one backyard cottage allowed on the same lot; parking no longer required for ADUs; the homeowner may live elsewhere.
  • Alternative 3: two ADUs allowed per lot (either two in-law apartments or one apartment and one cottage); an additional parking space is required only if two ADUs are built.

Alternatives 2 and 3 both reduce the minimum lot size required to build an ADU from 4000 square feet to 3200 square feet. Both action alternatives allow for slightly larger cottages and apartments than the no action alternative.

Alternative 3 also explores the option to include a size restriction on new single-family homes by implementing a floor area ratio limit. Today, the size of single-family homes is limited only by yard requirements, height limits, and lot coverage limits.

The no action alternative anticipates homeowners would build 1,890 ADUs by 2027. Alternatives 2 and 3 would both result in an increase in ADU construction in the next decade: 3,330 and 3,100 total new ADUs, respectively.

The Draft EIS was developed jointly by the Seattle City Council’s Central Staff and the Office of Planning and Community Development (OPCD). In December 2016, the Seattle Hearing Examiner ruled that a full EIS on Councilmember O’Brien’s proposal was required before changes could be adopted.

The Draft EIS identifies environmental impacts and mitigation measures for each alternative. Based on public comments during the public scoping phase in the fall of 2017 and direction from the Hearing Examiner, the analysis includes possible impacts related to housing and socioeconomics, land use, aesthetics, parking and transportation, and public services and utilities.

The City is now seeking public comment on the adequacy of this environmental review. A public comment period will end at 5:00 p.m. on June 25, 2018. The public can comment on the DEIS online at or by email to

The City will also hold an open house and public hearing at 5:30 p.m. on May 31, 2018 in the Bertha Knight Landes room at Seattle City Hall, where people may make verbal comments.

The City’s Final EIS and a preferred alternative of policy changes will be developed based on community comments already received and input on the three alternatives in the DEIS. Council Central Staff expects to complete the Final EIS in September before draft legislation is developed for the City Council’s consideration.