Oct 02, 2017
Seattle Department of Construction and Inspections
Land Use Information Bulletin
A Twice-Weekly Bulletin Announcing Land Use Applications, Decisions, Hearings, and Appeals

Area:     Address:
Project:     Zone: Notice Date: 10/02/2017

Project Name: Accessory Dwelling Units Environmental Impact Statement


Proponent: City of Seattle


Lead Agency: Seattle City Council


Project Location: City of Seattle


Description of the Proposal: The City of Seattle is proposing to change regulations in the Land Use Code to remove barriers to the creation of accessory dwelling units (ADUs) in single-family zones. ADUs include backyard cottages, known as Detached Accessory Dwelling Units (DADUs), and in-law apartments, known as Attached Accessory Dwelling Units (AADUs). The proposal involves allowing two ADUs on one lot, removing the existing off-street parking and owner-occupancy requirements, and changing some development standards that regulate the size and location of detached ADUs.


ADUs have been allowed citywide as part of a single-family house or in the backyard of a single-family-zoned lot since 1994 and 2010, respectively. The City’s action would modify the rules that regulate when and where a property owner can create an ADU. The objective of this action is to remove regulatory barriers to ADU production, increase the number of ADUs constructed in Seattle, and allow flexibility for larger ADUs that could accommodate changing household needs and families with children. These policy changes would affect development in Seattle’s single-family zones.


Environmental Determination: In May 2016, the City prepared an environmental checklist evaluating the potential environmental impacts of the proposed changes to the land use code. The environmental determination of non-significance in the checklist was appealed in June 2016. In December 2016, the Seattle Hearing Examiner determined that a more thorough review of the potential environmental impacts of the proposal was required (https://web6.seattle.gov/Examiner/case/W-16-004). Based on the Hearing Examiner’s decision, the Seattle City Council, as the SEPA lead agency, has determined that this proposal may have significant adverse environmental impacts on the environment. An EIS is required under RCW 43.21C.030 (2)(c) and will be prepared. The EIS will address probable significant adverse environmental impacts of the proposed changes to the Land Use Code.


Elements of the Environment: WAC 197-11 establishes that EISs shall provide a level of detail commensurate with the importance of the impact. The lead agency has preliminarily identified the following elements of the environment for analysis in the EIS:



Alternatives: The lead agency has preliminarily identified the following alternatives for analysis in the EIS. Table 1 compares these alternatives.



Table 1. Existing and Proposed Land Use Code Regulations for ADUs


Alternative 1 (No Action)

Alternative 2


Number of ADUs allowed on a single-family lot

A single-family lot can have one AADU or one DADU, but not both.

A single-family lot can have an AADU and a DADU.



One off-street parking space required for an AADU or DADU unless the lot is in an urban village.

No off-street parking required.



An owner must occupy either the main house or the AADU/DADU for six months of the year.

No requirement for an owner to occupy the house, AADU, or DADU.


Minimum lot size for DADUs

4,000 square feet

3,200 square feet


Maximum square footage

AADU 1,000 square feet, including garage and storage areas


DADU 800 square feet including garage and storage areas

AADU 1,000 square feet, excluding garage and storage areas


DADU 1,000 square feet, excluding garage and storage areas


Maximum height

No change from existing height limits, which vary by lot width and range from 15-23 feet.

Height limits are 1-3 feet higher than existing limits, depending on lot width.


Lot Coverage Limit

35 percent of lot area for lots 5,000 square feet and larger and 15 percent of lot area plus 1,000 square feet for lots under 5,000 square feet.


Rear yard coverage limit

40 percent of a rear yard can be covered by a DADU and other accessory structures (like a garage). This limit applies in addition to the overall lot coverage limit.

60 percent of a rear yard can be covered by a DADU and other accessory structures, if the DADU is only one story.


Location of entries

DADU entrances cannot face the nearest side or rear lot line unless that lot line abuts an alley or other public right-of-way.

DADU entrances can be on any façade, provided it is 10 feet from the lot line if located on the façades facing nearest side or rear lot line (unless abutting right-of-way).


Roof features

No exceptions for roof features on accessory structures are allowed.

Exceptions from the height limit are allowed for projections like dormers that add interior space, subject to the same provisions that govern exceptions for single-family houses.


Household Size

Any number of related people, or up to 8 unrelated people, can live on a single-family lot, including in an AADU or DADU.

Any number of related people, or up to 8 unrelated people, can live on a single-family lot with an AADU or DADU. If the lot has an AADU and DADU, the limit is 12.


MHA Requirements

Mandatory Housing Affordability (MHA) does not apply to creation of ADUs in Single Family zones.


Rental Registration and Inspection Ordinance (RRIO)

Property owners renting one or more units, including in Single Family zones, must register for inspections to ensure housing is safe and meets basic maintenance requirements.


Scoping: Agencies, affected tribes, and members of the public are invited to comment on the scope of the EIS within the 30-day comment period beginning on October 2, 2017, and closing on November 1, 2017. All comments are welcome, but comments in the following areas are particularly valuable:



All comments are due by 5:00 p.m., November 1, 2017, and may be submitted:


Meeting Locations:

October 17, 2017, 6:00-7:30 p.m.

High Point Community Center

6920 34th Ave SW

Seattle, WA 98126

October 26, 2017, 6:00-7:30 p.m.

Hale’s Ales (in the Palladium)

4301 Leary Way NW

Seattle, WA 98107


These meetings will provide an opportunity to learn more about the proposed land use code changes and provide input on the environmental review process.


Date:  October 2, 2017       SEPA Responsible Official:   ___on file_______________

                                                                               Ketil Freeman, AICP

                                                                               City Council Central Staff