Accessory Dwelling Units EIS

Thanks for your suggestions about what to consider in our study.

On October 2, we began the environmental review process to study the effects of removing barriers to creating accessory dwelling units (ADUs), often called in-law units and backyard cottages, in single-family zones. The first phase of the Environmental Impact Statement (EIS) process is to determine the scope of our analysis. We took scoping comments on our alternatives and proposed scope from October 2 to November 16, including two public scoping meetings. We’re now reviewing feedback and will summarize comments in a scoping report.

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. 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 objectives of this action are to

  • Remove regulatory barriers so it's easier for property owners to permit and build ADUs and backyard cottages
  • Increase the number and variety of housing choices available in single-family zones
  • Encourage creation of small-scale, family-friendly homes affordable to a range of households and flexible for their changing needs

These policy changes would affect development in Seattle’s single-family zones.

Based on a decision from the City’s Hearing Examiner in December 2016, we are preparing an Environmental Impact Statement (EIS) that will analyze two alternatives and identify the impacts of each alternative.

Get Involved

UPDATE: We've extended the comment period to November 16

Comment on the proposed EIS scope  

What is an ADU?

ADUs are small secondary dwelling units inside, attached to, or in the rear yard of a single-family house. An attached ADU (AADU), often called an in-law unit or a granny flat, is contained within or attached to a single- family house. A detached ADU (DADU), often called a backyard cottage, is a separate structure allowed in the rear yard of certain single-family-zoned lots. DADUs can be new structures or created through conversion of an existing structure, like a garage.

What is an EIS?

An EIS is a tool to inform decision makers about the positive and negative effects of a proposal. The proposal might be a project, like construction of a new building or road, or a new policy or plan that could affect the environment. Washington’s State Environmental Policy Act (SEPA) requires Environmental Impact Statements so that the public, tribes, and other public agencies can help identify a proposal’s environmental impacts, as well as strategies for reducing or avoiding them. Decision-makers can then approve, modify, or deny the proposal as appropriate.


From October 2 to November 16, we took input on what we should study in the EIS. This scoping period included two public scoping meetings on October 17 and 26 at the High Point Community Center and Hale’s Ales.

Watch a video presentation about the scoping process:


Issue Determiniaton of Significance & Scoping Notice
Determination of Significance and Scoping Notice for the ADU EIS was issued on
October 2, 2017
Conduct SEPA Scoping
The scoping comment period will close at 5:00 p.m. on
November 1, 2017
Extended to November 16, 2017
Prepare Draft EIS
Scoping comments will be reviewed and the Draft EIS will be prepared
Issue Draft EIS
Tentative issuance Spring 2018
Draft EIS Public Comment Period
A 30-day comment period will follow the issuance of the Draft EIS and will include a public hearing
Prepare Final EIS
The Final EIS will address comments received during the comment period
Issue Final EIS
Tentative issuance Summer 2018
City Action
The City Council will vote on proposed legislation to amend the Land Use Code

Comparison of Alternatives

During the scoping period, we invited the public to comment on the following proposed alternatives:

Alternative 1 (No Action)Alternative 2
1 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.

2 Parking

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

No off-street parking required.

3 Owner-occupancy

An owner must occupy either the main house or the AADU/DADU 6 months a year.

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

4 Minimum lot size for a DADU

4,000 square feet

3,200 square feet

5 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

6 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.

7 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.

8 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 and if rear yard coverage from other accessory structures is less than 40 percent.

9 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).

10 Roof features

No exceptions from the height limit are allowed for roof features on accessory structures.

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

11 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.

12 MHA requirements

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

13 Rental Registration & 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.

Proposed Scope

The EIS will incorporate information and analyses from the Mandatory Housing Affordability (MHA) EIS (2017), the Seattle 2035 Comprehensive Plan EIS (2016), the Growth and Equity Analysis (2016), and other recent City studies and plans.

In December 2016, the Seattle Hearing Examiner determined that a thorough review of the proposal’s potential environmental impacts through an EIS was necessary. Based on this decision, we have preliminarily identified the following elements of the environment for analysis in the EIS:

Land Use

  • Compatibility of alternatives with single-family zoning
  • Potential elimination of existing housing
  • Potential impacts on vegetation, tree canopy, and environmentally critical areas (ECAs)

Housing & Socioeconomics

  • Feasibility of development scenarios
  • Housing affordability
  • Assessment of socioeconomic characteristics, demographic change, and potential displacement


  • Potential impacts to visual character
  • Qualitative review of shadowing, privacy, scale, and compatibility with single-family development


  • Potential impacts to availability of on-street parking
  • Assessment of car ownership rates, transit, and circulation patterns

Public Services & Utilities

  • Police, fire and emergency services, public schools, water, sewer, stormwater
  • Potential impacts on demand for services in each alternative

Study Area

Map of the ADU EIS study area


Scoping Handout

Scoping Meeting Boards

Scoping Comments – As of 10/30/17*

Presentation from 10/17 and 10/26 Public Scoping Meetings

*Note: we are still reviewing comments received to date and may not have posted every comment received to date; all comments submitted will be posted with the final scoping report.