Backyard Cottages and Basement Units


On July 1, 2019 the Council unanimously passed legislation (Council Bill 119544) that will make it easier for more property owners to build backyard cottages (detached accessory dwelling units or DADUs) and basement units (attached accessory dwelling units or AADUs) and therefore provide more housing options for people living in Seattle.

The legislation:

  • Reduces the minimum lot size required to build a DADU on a single-family lot from 4,000 square feet to 3,200 square feet;
  • Increase the maximum size of DADUs from 800 square feet to 1,000 square feet, excluding any parking or storage areas;
  • Removes the owner-occupancy requirement for ADUs;
  • Removes the off-street parking requirement for ADUs;
  • Allows two ADUs on one lot (either one attached and one detached, or two attached) if the second ADU meets a green building standard or will be affordable to households at or below 80% of area median income;
  • Increases the maximum household size permitted on a single-family lot from 8 to 12 unrelated people only if the lot includes two ADUs;
  • Increases DADU height limits by 1-3 feet, with flexibility for green building strategies;
  • Allows design flexibility to preserve existing trees and to convert existing accessory structures to a DADU;
  • Require annual reporting on ADU production and requires that the Office of Planning and Community Development and the Seattle Department of Construction and Inspections conduct a survey of ADU owners and occupants within 3 years.
  • Introduces a Floor Area Limit (FAR) for all new development in single-family zones with some exemptions (this regulation has a delayed effective date until March 1, 2020);

Look for updates to the Seattle Department of Construction and Inspections’ accessory dwelling unit webpage for more information after the new regulations go into effect in early to mid-August.

Policy Objectives

Affordable housing is one of the biggest challenges we face in Seattle today. City leaders have identified the need for providing a mix of housing types at prices people at all levels of income, both for homeowners and renters, can afford. Increasing the number of accessory dwelling units (ADUs) in single-family zones is part of addressing our housing affordability crisis.  ADUs include both backyard cottages or Detached Accessory Dwelling Units (DADUs) and in-law or basement apartments or Attached Accessory Dwelling Units (AADUs).  ADUs have the potential to provide new housing opportunities in neighborhoods where single family homes are often unaffordable to many people. If just 5% of eligible lots in the city build ADUs it would create about 4,000 housing units.

For more than a decade we have been working to identify barriers to production of ADUs and propose solutions beneficial to homeowners and renters alike.  My policy objectives are: 

  1. increase ADU production to grow the volume and variety of housing choices available in single-family zones;
  2. support lower- and middle-income homeowners in developing ADUs; and
  3. increase access to ADUs for lower income renters.

To accomplish these goals, we must address challenges in our existing Land Use Code that create barriers to ADU production. 

A backyard cottage in Ballard

A backyard cottage in Ballard

Current Status

Land Use Code Changes: A little history...

In May of 2016, the Seattle City Council released a proposal that would make it easier for more homeowners to build backyard cottages and basement units and therefore provide more housing options for Seattle renters.  The environmental review for that proposal was appealed. 

Based on a decision from the City’s Hearing Examiner in December 2016, the Council was required to prepare an environmental impact statement (EIS) describing potential environmental impacts of proposed changes to the Land Use Code intended to remove barriers to building ADUs. Following that Decision, the City underwent the process of selecting a consultant team and preparing the EIS. On October 4, 2018, the Final EIS was issued.  The EIS identifies potential environmental impacts of the proposed code changes and will be inform.

The Preferred Alternative described in the EIS will be the basis for my legislation moving forward.  There are a few key issues that differ from my original proposal, that after the full analysis of the EIS, I intend to integrate.  I have received many questions and heard a lot of interest in these key changes.  This is the beginning of the conversation, and I want to share my thinking (see below).  I look forward to receiving feedback on an ongoing basis as we move forward with the legislative process.

  • Floor Area Ratio (FAR) limits are often used to regulate the size of a building. Floor area ratio is the ratio of a building’s total square footage (floor area) relative to the size of the piece of land on which it is constructed. Under current regulations, there is not a maximum FAR limit in single-family zones. Under the proposal, a maximum FAR limit would govern the size and scale of development of new homes in single-family zones, along with maximum height limit, maximum lot coverage limit, and yard requirements. More information about the proposed FAR requirement to limit the construction of very large houses (i.e. McMansions) is available here.
  • Owner occupancy, under current regulations, a property owner is required to occupy either the main house or the accessory dwelling unit (ADU) for six months of the year. They cannot rent out both the main house and the ADU. The current proposal would remove the owner-occupancy requirement. The proposal would allow a second ADU only if the property has been owned by the same person/entity for at least one year. More information about removing this requirement is available here.

On October 18, 2018, an appeal of the adequacy of the Final EIS was filed with the City of Seattle Hearing Examiner. This appeal process is not where decisions are made about forthcoming legislation, but where the Examiner evaluates the adequacy of our analysis.  The hearing has been set for the week of March 25th (2019), with a decision expected soon.  That decision will determine if more analysis is needed for the EIS, or if the EIS is adequate such that the Council can move forward with legislation.  Assuming a positive outcome from the appeal, my plan is to introduce legislation based on the Preferred Alternative  as a starting point for our legislative process by June 2019.

I believe changes to the Land Use Code will lower the barriers to creating backyard cottages and basement apartments and create more housing choices in single-family zones. I see this as an important part of addressing affordability across the city and am looking forward to passing this legislation.

Supporting low-income homeowners and renters to access ADUs

My office recently completed a Racial Equity Toolkit (RET) to assess the racial equity impacts of proposed policies, programs, and investments. You can view the presentation and discussion held during a special meeting of the Sustainability and Transportation committee about the RET.

The analysis conducted for the RET highlighted that Land Use Code changes alone are insufficient to address racial disparities that have resulted from a history of race- and class-based planning and housing policies. This is due, at least in part, because absent other policy interventions, wealthy, primarily White homeowners are most likely to have access to the capitol needed to construct an ADU. Further, because of the high cost of construction, while ADUs may rent at lower price points than a traditional single family home due to the smaller size, they are still typically priced above what households with lower-incomes and households of color can afford.   That is why my office is working to develop additional tools to:

  • Support low-income homeowners and homeowners who are people of color to stay in their communities—with tools to help leverage the value of their property without selling, by creating additional living space or ADUs for family members, or to generate rental income.
  • Create rent- and income-restricted ADUs so low-income households can benefit from new housing opportunities in single-family zones.

Based on what we have learned to date through the RET process, we are pursuing the following strategies in 2019:

  1. Pilot program to create more habitable space
    Currently, the Seattle Office of Housing (OH) runs a home repair program that provides low interest loans or grants (depending on qualification) to low-income homeowners to address critical health and safety concerns. We plan to expand this program to allow for property improvements that create additional habitable space within the existing “envelope” of the property. This could include finishing a basement, creating an AADU, converting a garage to a DADU, or bringing an existing unpermitted ADU up to code. These improvements could allow a homeowner to house additional family members or generate additional rental income. In its initial year, we hope to serve five to ten low-income homeowners and build a better understanding of the needs, project costs, and challenges, of helping people stay in their homes. The intent is to support homeowners at risk of displacement to leverage the value of their property without having to sell, while avoiding assuming the level of risk or debt that is typically required for these types of improvements.
  2. Community outreach resources
    From our focus groups conducted as part of the RET, we learned that while the City offers multiple programs through OH that support low-income homeowners to stay in their homes, many people who are eligible for these programs do not know about them. We added additional resources to the OH budget to contract with community-based organizations to share information about existing, and possible future programs, which will help ensure we meet our racial equity goals.
  3. Urban Sustainability Accelerator
    Through September 2019, the Office of Planning and Community Development is leading an interdepartmental team with representatives from the Seattle Department of Construction and Inspections, Office of Housing, Planning Commission, and Council Staff, to participate in the Urban Sustainability Accelerator program, a year-long cohort of city and county teams from across the country working to promote ADUs. The teams work will focus on programmatic ideas that align with addressing the challenges we hear from homeowners and will prioritize programs that further racial equity.

I am hopeful that with thoughtful investments, homeowners of all income levels can benefit from the city’s investments in creating more housing across single family zones.

If you have further questions, please reach out to Alisha Dall'Osto – alisha.dall' or call our office at 206-684-8800.

Summary of Proposed Changes in Preferred Alternative

Policy AreaHow It Is TodayCM O’Brien’s 2018 Proposal (Preferred Alternative in the Final EIS)CM O’Brien’s 2016 Proposal
Number of ADUs allowed on lots in single-family zones A lot with or proposed for a single-family dwelling may have no more than one accessory dwelling unit. Allow a property to have two ADUs (either an AADU and a DADU or two AADUs). A second ADU can be added if a lot has been in the same ownership for at least one year Allow a property to have both an AADU and DADU within the existing building envelope.
Off-street parking requirement for ADUs One off-street parking space is required for the ADU, unless located in an urban center or urban village. No off-street parking required for ADUs.
Owner-occupancy requirement An owner must occupy either the principal dwelling unit or the accessory dwelling unit. No requirement for an owner to occupy the principal dwelling unit, AADU, or DADU. Require owner-occupancy for 1 year, then requirement expires
Minimum lot size for a DADU The current minimum lot size for a site with a DADU is 4,000 square feet. Reduce the minimum lot size for a site with a DADU to 3,200 sq. ft.
Maximum square footage of a DADU The maximum gross floor area of a DADU is 800 square feet including any garage and storage areas. The maximum gross floor area of an ADU is 1,000 square feet. Increase the maximum gross floor area of a DADU to 1,000 square feet; exclude garage and storage areas.
Minimum lot size to create a new single-family lot No change from current regulations:
-   SF 5000 5,000 sq. ft.
-   SF 7200 7,200 sq. ft.
-   SF 9600 9,600 sq. ft.
No change proposed
Height limit for a DADU The maximum height limit a DADU depends on the width of the lot; on wider lots, a taller DADU is permitted (see Table A below) Increase maximum height by 1-2 feet (See Table B below) and allow 1 to 2 additional feet for a DADU that incorporates green building strategies. Increase maximum height by 1-2 feet and simplify code (See Table B below)
Rear yard coverage limit A maximum of 40% of a rear yard may be covered by accessory structures and any portion of the main house. This limit is in addition to the overall lot coverage limit for a single-family lot Increase rear yard coverage to 60% for a DADU whose total height is no more than 15 feet, subject to limitations on tree removal. Rear yard coverage for structures other than a DADU cannot exceed 40 percent. Increase rear yard coverage to 60% for one-story DADUs
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. Allow DADU entrances 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 Exceptions for roof features for a accessory units are not permitted. Height limit exceptions are allowed for projections like dormers that add interior space, subject to the provisions applicable to single-family houses
Maximum household size Any number of related people, or up to eight unrelated people, can live on lots in single-family zones including in an AADU or a DADU. Any number of related people, or up to eight unrelated people, can live on lots in single-family zones with an AADU or a DADU. If the lot has two ADUs, the limit is 12 unrelated people. No changes proposed
Maximum floor area ratio (FAR) limit No FAR limit for single-family zones. The maximum size for the main house is effectively set by the yard requirements, height limit, and lot coverage limit. ADUs are subject to the maximum size limits described above. New construction

FAR limits apply to development in single-family zones. New houses (i.e., principal structures) are subject to an FAR limit of 0.5 or 2,500 square feet, whichever is greater. Below-grade floor area and floor area in an ADU is exempt.

Existing houses

Existing lots in single-family zones exceeding the FAR or 2,500-square-foot limits can convert existing space to an AADU and add a DADU subject to the size limit above.

No change proposed

Height Limits for DADUs

Table A - Current

Lot width (ft)<3030-3535-4040-50>50
Base height (ft) 12 14 15 16 16
Additional height for pitched roof (ft) 3 7 7 6 7
Additional height for shed/butterfly roof (ft) 3 4 4 4 4

Table B - Proposed

Lot width (ft)<3030-50>50
Base height (ft) 14 16 18
Additional height for pitched roof (ft) 3 7 7
Additional height for shed/butterfly roof (ft) 3 4 4

Personal Stories from Backyard Cottage Owners

"My backyard cottage is for my Mom, who at 80 years old needs to be close but is not ready for assisted living."

"Our original intent was a glorified garage with electrical and plumbing, as a short-term place for friends to stay,. The permitting process forced us to expand the project into a more traditional living space. Now that it is a full blown space (kitchen with range, washer / dryer, loft, etc.) we love the rental income (from a long-term tenant)."

"My neighbors really like the cottage and often stop by to say so, or ask how they might build one.. One reason it worked well for me is that I already have a very small house (650 sq ft) and adding the cottage doesn't overwhelm the space."

"I was looking for two houses on one lot when I bought this house. I needed the second income as a single woman."

Resources and Materials

Proposal Encouraging Backyard Cottages and Basement Units

Councilmember Mike O'Brien (District 6, Northwest Seattle) released a proposal on May 19, 2016 that would make it easier for more homeowners to build backyard cottages and basement units in Seattle. As the city faces an affordable housing crisis, Councilmember O'Brien developed the legislation to increase the housing supply by encouraging low-impact housing options that fit within the scale and character of Seattle's single-family neighborhoods.

Councilmember O'Brien's bill makes a series of changes to the existing backyard cottage and basement unit building code, including:

  • Allowing both a backyard cottage and basement unit on the same property, which maintains the character and appearance of the property while providing an additional housing option.
  • Increasing the height limit for backyard cottages by 1-2 feet, depending on lot width, which would create enough livable space to make two-bedroom units more feasible. Setback requirements from property edges would not change.
  • Removing the requirement for owners to include an off-street parking space for backyard cottages or basement units, which prevents removal of green space on the property. Feedback found the parking requirement was prohibitive in creating new backyard cottages, as additional parking spaces were either unnecessary or unable to fit on the lot. For single-family lots outside urban villages or urban centers, the one parking space per house requirement will still apply.
  • If a backyard cottage is only one-story, its floor area may cover up to 60% of the rear yard (currently 40%), creating a large enough livable space for those unable to use stairs. Existing setback requirements from the lot edge would not change.
  • Requiring that the property owner live on-site for at least one year after a backyard cottage or basement unit is created, rather than the current requirement that the owner live on-site at least 6 months out of every year in perpetuity. The requirement prevents speculative developers from acquiring property and building backyard cottages that don't fit the character of the neighborhood, while allowing the owner future flexibility for those who don't want, or are unable to, continue living on-site.
  • Allowing backyard cottages on lots 3,200 square feet or greater (currently 4,000 square feet), which would make approximately 7,300 additional parcels eligible to provide this additional housing option.
  • Increasing the maximum gross floor area of a backyard cottage to 1,000 square feet (currently 800 square feet), which would provide more livable area and increase the likelihood of two-bedroom backyard cottages to better serve families with children.
  • If a backyard cottage is built above a garage, the garage square footage will no longer count toward the maximum floor area, as it might result in an unreasonably small living space.

Removing barriers to backyard cottages and accessory dwelling units

Read more about the proposed legislation in the Director's Report

"With these amendments to the existing code, we could see thousands of new housing units that simultaneously fit into the context of a neighborhood and serve property owners," said Councilmember O'Brien. "By expanding the availability of backyard cottages and basements, someone might be able move to a neighborhood they otherwise couldn't afford while helping a homeowner who needs an extra source of income to afford to stay where they are. It's a win-win."

The legislation was developed utilizing feedback from neighborhood community meetings, from architects, and from current backyard cottage owners. A summary of the public feedback is available here. The former Seattle Department of Planning & Development also conducted an analysis of current backyard cottages in Seattle and a review of peer cities' backyard cottage model.

Backyard cottages, also called detached accessory dwelling units (DADUs), are separate living spaces on the same property as an existing single-family house. basement units, also referred to as accessory dwelling units (ADUs), are separate living units generally located within a single-family house.