If you were beginning to think backyard cottages were going to end up on the legislative agenda backburner, think again. Councilmember Mike O’Brien has formally unveiled his legislation to remove barriers from building backyard cottages in Seattle. During the review of backyard cottage housing, O’Brien and staff from the Office of Planning and Community Development conducted extensive public outreach to get feedback on issues like requirements for off-street parking, owner occupancy, height limits, and maximum square footage of units.

Known more formally as Detached Accessory Dwelling Units (DADUs), the City has permitted them on a widespread basis since 2010 and has experienced very low interest in development due to regulatory requirements. In total, 221 individual DADUs have been built. The City estimates that some 4,000 DADUs could be built if property owners of just 5% of eligible lots decided to pursue development of these units.

For the wonks out there, Councilmember O’Brien put together a helpful side-by-side comparison of existing code versus proposed regulatory changes:

Summary of Proposed Changes

#Policy AreaHow It Is TodayProposed Change
1Attached Accessory Dwelling Unit (ADU) and Detached ADU (DADU) on the same lotA 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; and

  • 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)
2Off-street parking requirementOne 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; and

  • Removing this requirement will allow some properties who have insufficient space for an additional off-street parking space, to add an ADU or DADU.
3Owner-occupancy requirementAn 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 homebuyers who can use rental income from ADU/DADU; and

  • Alleviates some of lenders' concern with financing.

4Minimum lot size for DADUsThe 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 square feet:

  • Approximately 7,300 more lots become eligible to build a DADU; and

  • Development standards like lot coverage limits, yards, and setbacks would continue to apply and would govern the footprint, scale, and location of a DADU.
5Maximum square footage of a DADUThe 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; and

  • Other development standards would continue to regulate scale of DADUs (lot coverage, height limit, etc.).
6Rear yard coverage limitA 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; and
  • Does not increase the overall lot coverage limit for a single-family lot

7Location of entriesDADU 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; and

  • Continues to address privacy concerns.
8Height limitThe 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; and

  • Facilitates family-size units.
9Roof featuresExceptions 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); and

  • Additional interior space valuable for DADUs.
Comparative table for allowable height limits of DADUs. (City of Seattle)
Comparative table for allowable height limits of DADUs. (City of Seattle)

Councilmember Mike O’Brien’s backyard cottage legislation should be introduced to the Council soon for further consideration.

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Article Author

Stephen is a professional urban planner in Puget Sound with a passion for sustainable, livable, and diverse cities. He is especially interested in how policies, regulations, and programs can promote positive outcomes for communities. With stints in great cities like Bellingham and Cork, Stephen currently lives in Seattle. He primarily covers land use and transportation issues and has been with The Urbanist since 2014.