Accessory dwelling units

Authorize attached accessory dwelling units on all parcels containing single-family homes where the lot is at least three thousand two hundred square feet in size, and permit both attached and detached accessory dwelling units on all parcels containing single-family homes, provided lots are at least four thousand three hundred fifty-six square feet in size. Qualifying city ordinances or regulations may not provide for on-site parking requirements, owner occupancy requirements, or square footage limitations below one thousand square feet for the accessory dwelling unit, and must not prohibit the separate rental or sale of accessory dwelling units and the primary residence. Cities must set applicable impact fees at no more than the projected impact of the accessory dwelling unit. To allow local flexibility, other than these factors, accessory dwelling units may be subject to such regulations, conditions, procedures, and limitations as determined by the local legislative authority, and must follow all applicable state and federal laws and local ordinances.

Draft Codes

Adopted Codes

Other Code Examples

Since many Washington cities and counties are required to have allow accessory apartments and ADUs, this section of resources focuses on existing resources and those examples that are most similar to the requirements of HB 1923 regulations.

  • MRSC: Accessory Dwelling Units webpage includes information on:
    • What is an accessory dwelling unit (ADU),
    • ADUs in Washington and when they are required in a jurisdiction (including a link to model ordinance recommendations),
    • Examples of Local Jurisdictions
  • MRSC: Accessory Dwelling Units: Issues & Options report (1995)
  • MRSC: Encouraging Residential Infill (see Accessory Dwelling Unit section)
  • Black Diamond allows accessory dwelling units with minimal performance standards in any residential district (scale, utility connections, entrance locations).  ADUs may be up to 1,000 SF in size, but ADUs must not be segregated in ownership from the principal dwelling and must have one parking space (Chapter 18.56).
  • Seattle allows ADUs without parking and without owner occupancy, and units may be up to 1,000 SF in size (44.041 Accessory dwelling units). In addition to the normal setback and coverage requirements, the only design requirements are related to height and entrance location.
  • Vancouver allows ADUs without parking and without owner occupancy (810 Accessory Dwelling Units). ADUs, however, may not exceed 800 SF or 50% of the primary dwelling unit, whichever is less. Minimal design requirements.
Other Resources and Information

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