Backyard Cottages


Affordability in housing is one of the biggest issues we face in Seattle today. The City of Seattle has identified the need for providing a mix of housing types at prices accessible to people at all levels of income, both for homeowners and renters. Backyard cottages, or Detached Accessory Dwelling Units (DADUs,) and in-law apartments, or Attached Accessory Dwelling Units (AADUs), have the potential to provide a significant amount of more affordable options for housing in neighborhoods where homes are often unaffordable to many people. If just 5% of eligible lots in the city build DADUs it would create about 4,000 housing units. There are relatively few backyard cottages in Seattle right now. 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 in Seattle and provide more housing options for Seattle renters.

A backyard cottage in Ballard

A backyard cottage in Ballard

Current Status

Land Use Code Changes

Based on a decision from the City’s Hearing Examiner in December 2016, we prepared an environmental impact statement that describes potential environmental impacts of proposed changes to the Land Use Code Code intended to remove barriers to building ADUs. Published on October 4, 2018, the Final EIS includes a Preferred Alternative that represents the changes I plan to include in legislation for consideration in 2019. The Final EIS enables us to look deeply into the potential environmental impacts of the proposed code changes and inform the final proposal.

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 continuing to pursue this legislation.


While I believe code changes will lower the barriers for many homeowners, I also recognize that without additional strategies to support low-income homeowners and renters and homeowners and renters of color, flexibility for creating ADUs will disproportionately benefit homeowners who tend to be wealthy and White. That is why my office is working to develop additional tools to:

  • Support low-income homeowners and homeowners of color to stay in their communities—with tools to help leverage the value of their property without selling, to create additional units for family members or to generate rental income;
  • Create rent- and income-restricted backyard cottages and basement units so low-income individuals and families can benefit from new housing opportunities in single-family zones.

We are currently conducting a Racial Equity Toolkit to assess the racial equity impacts of proposed policies, programs, and investments. This work will inform development of additional programmatic strategies to advance opportunity and minimize harm for communities of color and low-income homeowners and renters. Some of the programmatic ideas the City is considering include:

  • Providing homeowners with better access to financing
    • Explore partnering with non-profit lenders to lower interest rates and cover risk
  • Creating resources to help homeowners explore their options for creating an additional unit
    • Developing pre-approved backyard cottage designs to save homeowners time and money in the design and permitting process
    • Working with industry experts to develop standardized designs to reduce construction costs
    • Assisting homeowners in navigating the permitting and construction process
    • Providing training and resources to help homeowners become landlords
  • Requiring rent-restricted ADUs for income-qualified renters along with these benefits

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 Susie Levy – or call our office at 206-684-8800.

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.

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."

Summary of Proposed Changes

#Policy AreaHow It Is TodayProposed Change
1 Attached Accessory Dwelling Unit (ADU) and Detached ADU (DADU) on the same lot A lot with or proposed for a single-family dwelling may have no more than one accessory dwelling unit. Allow a property to have both an ADU and DADU within the existing building envelope

  • Does not change the total size of buildings on a lot and development standards like lot coverage limits, yards, and setbacks would continue to apply
  • Does not increase the total number of people who can live on the lot (if unrelated persons occupy any of the units, the total number of persons occupying all units may not altogether exceed eight)
2 Off-street parking requirement One off-street parking space is required for the accessory dwelling unit, unless located in an urban center or urban village. Remove requirement for off-street parking

  • A property owner would still be able to provide an off-street parking space if they have space and believe it is a desirable amenity for prospective tenants
  • Removing this requirement will allow some properties who have insufficient space for an additional off-street parking space, to add an ADU or DADU
3 Owner-occupancy requirement An owner must occupy either the principal dwelling unit or the accessory dwelling unit. Require owner-occupancy for 1 year, then requirement expires

  • Maintains current dynamic of homeowners designing/creating ADU/DADUs
  • Addresses some homeowners' concerns by preventing landlords and investors from creating ADU/DADUs
  • Gives flexibility to homeowners to move after 1 year, rent both units
  • Increases homeownership options for prospective home buyers who can use rental income from ADU/DADU
  • Alleviates some of lenders' concern with financing
4 Minimum lot size for DADUs 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.

  • Approx. 7,300 more lots become eligible to build a DADU
  • Development standards like lot coverage limits, yards, and setbacks would continue to apply and would govern the footprint, scale, and location of a DADU
5 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

  • Consistent maximum square footage for ADUs and DADUs
  • Facilitates family-size units
  • More flexibility for DADUs over garages
  • Other development standards would continue to regulate scale of DADUs (lot coverage, height limit, etc.)
6 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 one-story DADUs

  • More flexibility for one-story designs, which are:
    • less visible than two-story DADUs
    • better for limited-mobility tenants and aging-in-place
  • Does not increase the overall lot coverage limit for a single-family lot
7 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)

  • Adds design flexibility
  • Addresses challenge we've heard from architects
  • Continues to address privacy concerns
8 Height limit The maximum height limit 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 and simplify code (See Table B below)

  • Makes 2-bedroom DADU feasible on more lots
  • Facilitates family-size units
9 Roof features Exceptions for roof features for a accessory units are not permitted. Allow exceptions from height limit for projections (e.g., dormers) that add interior space.

  • Same provisions as single-family houses (e.g. limits on roof coverage)
  • Additional interior space valuable for DADUs

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