On June 10, 2025, City Council adopted Ordinance #25-1008, adopting and amending SeaTac Municipal Code (SMC) sections related to middle housing and Accessory Dwelling Unit (ADU) regulations, effective June 21, 2025.

New Development Regulations Effective June 21, 2025

• SMC 15.105: Definitions
• SMC 15.110.030: Designation of Lot Lines
• SMC 15.205.040: Land Use Chart
• SMC 15.300.055: City Center Overlay District Use Chart
• SMC 15.305: S 154^th St Station Area Overlay District
• SMC 15.310: Angle Lake Station Area Overlay District
• SMC 15.400: Dimensional Standards and Regulations
• SMC 15.405: Accessory and Tent Structures
• SMC 15.425: Development Incentives
• SMC 15.445: Landscaping and Tree Retention
• SMC 15.455.120: Parking Chart for Required Off-Street Spaces
• SMC 15.465: Residential Standards and Regulations
• SMC 15.500: Small Lot Detached Dwelling Unit Design Standards
• SMC 15.505: Townhouse Development Design Standards
• SMC  15.510: Multi-Family Housing Design Standards 

New legislation (HBs 1110 and 1337) passed in 2023 and 2024 requires all cities planning under the Growth Management Act (GMA) to make certain changes to codes and development regulations to allow middle housing and Accessory Dwelling Units (ADUs). While SeaTac has already made some of these changes through Envision SeaTac 2044, more must be done to implement the new state requirements. Additionally, the City can consider taking action that goes beyond the minimum requirements.

HB 1110 updated the GMA, mandating that certain GMA planning cities include middle housing types at minimum densities within six months after the periodic update due date. It also requires that specific provisions be made for middle housing in their development regulations. HB 1337 is a statewide housing bill focused on ADUs, aiming to broaden housing choices by easing barriers to the construction and use of accessory dwelling units. The bill also addresses topics such as ADU size standards, owner occupancy, and condominium sales.

Washington State defines middle housing, in the Growth Management Act (GMA), as “…buildings that are compatible in scale, form, and character with single-family houses and contain two or more attached, stacked, or clustered homes including duplexes, triplexes, fourplexes, fiveplexes, sixplexes, townhouses, stacked flats, courtyard apartments, and cottage housing.” These residential developments are more dense than single-family housing and less dense than typical midrise apartment buildings, but similar in scale to single-family homes.

Examples of some middle housing types defined in HB 1110. Top center and top right images courtesy of Sightline, other images by MAKERS.

HB 1110’s requirements include:

• Number of units allowed per lot:
  • Allow up to two housing units per lot on all lots zoned predominantly for residential use
  • Allow up to four housing units per lot on all lots zoned predominantly for residential use within one-quarter walking distance of a major transit stop
  • Allow up to four housing units per lot on all lots zoned predominantly for residential use if at least one unit is affordable
• Allow at least six of the nine middle housing types
• Parking
  • The City cannot require off-street parking as a condition of permitting middle housing within one-half mile walking distance of a major transit stop
  • The City cannot require more than one off-street parking space per unit as a condition of permitting middle housing on lots smaller than 6,000 sq. ft. before any zero lot line subdivisions or lot splits;
  • The City cannot require more than two off-street parking spaces per unit as a condition of permitting middle housing on lots greater than 6,000 sq. ft. before any zero lot line subdivisions or lot splits;
• Cannot require any standards for middle housing that are more restrictive than those required for detached single-family residences.

ADUs are subordinate dwelling units added to, created within, or detached from a principal dwelling unit, providing independent living facilities that include permanent provisions for living, sleeping, eating, cooking, and sanitation. An attached ADU is a dwelling unit located within or attached to another housing unit, while a detached ADU is separate and unconnected to the other housing unit.

HB 1337’s requirements include:

• Must allow at least two ADUs on all lots that are located in all zoning districts within an urban growth area that allow for detached dwelling units, in the following configurations:
  • One attached ADU and one detached ADU
  • Two attached ADUs
  • Two detached ADUs, which may be comprised of either one or two detached structures
• Cannot establish a maximum gross floor area requirement for ADUs that is less than 1000 square feet
• Must allow ADUs to be converted from existing structures, including but not limited to detached garages, even if they violate current code requirements for setbacks or lot coverage
• Cannot require off-street parking as a condition of permitting ADUs within one-half mile walking distance of a major transit stop
  • Does not apply within a mile radius of SeaTac International Airport

In order to implement these requirements, several different parts of the SeaTac Municipal Code (SMC) are proposed to be revised. SeaTac has flexibility in how its development regulations accommodate these units to encourage middle housing development. These areas of flexibility include requirements for:

• Building height
• Setbacks
• Lot coverage
• Minimum lot size
• Parking flexibilities
• Priorities for site design for cottage housing:
• Common space
• Landscaping

Meeting Information

View meeting packet information for the Planning Commission (PC), Planning & Economic Development (PED), or Council Committee Meetings by clicking on the appropriate link provided in the table below.

Draft Code

Public Hearing Draft Code

Additional Information

Planning for Middle Housing – Washington State Department of Commerce

ADU_Guide_Revised_6-24.pdf | Powered by Box

 Contact Information

Laura Stilwell, Associate Planner

206-973-4833

lstilwell@seatacwa.gov