Accessory Dwelling Units (ADUs)

An Accessory Dwelling Unit (ADU) is an residential structure used in connection with, or accessory to, a single-family dwelling. These homes, sometimes referred to as “mother-in-law apartments” or “granny flats”, are smaller than the primary dwelling but serve as complete independent living units with kitchens, bathrooms and sleeping areas. An ADU may be located within, attached to or detached from the primary dwelling.

ADU Illustrations


Growing concern for housing affordability in the Portland region and nationwide has raised the profile of ADUs in recent years. Some homeowners seeking flexible, affordable housing options for their families have turned to ADUs as a solution. Planners and policymakers have also seen ADU development as one way of potentially increasing housing supply with minimal visual impact in single-family neighborhoods. ADU development can improve housing supply and affordability by:

  • Creating smaller units that translate into more affordable rents (ADUs are generally limited in size – a maximum of 800 square feet is common among jurisdictions in Oregon)
  • Enabling seniors or adult children on limited incomes to more easily live on-site with other family members
  • Providing property owners with added income, thereby also assisting with the affordability of the primary dwelling
  • Offering the potential to increase residential supply even in low-density districts of the County, to help reduce inflated housing costs that come from undersupply

The State of Oregon also recognizes ADU development as an opportunity to improve housing supply and affordability. In 2017, the State legislature passed Oregon Senate Bill 1051, requiring certain cities and counties to allow ADUs in all areas within the Urban Growth Boundary zoned for detached single family dwellings. Furthermore, the Oregon Department of Land Conservation and Development (“DLCD”) recommended additional code changes that could make it easier for homeowners to build ADUs, including relaxing siting and design requirements.



Washington County has formally allowed ADUs since 1983. The ADU code remained largely unchanged until 2000 when County staff made revisions to allow ADU development in more residential land use districts, remove the minimum required floor area for primary dwellings to have ADUs (1500 sq. ft.) and add detached ADUs as an eligible use.

In 2013, LUT revised the ADU code again to change them to a Type I use and to allow up to a 15 percent increase in size if an ADU meets Americans with Disabilities Act (ADA) standards to accommodate people with mobility impairments. In 2018, the County made further changes to comply with new state laws (Oregon Senate Bill 1051), including removing design standards that weren’t clear and objective and allowing ADUs in all residential land use districts that allow detached single-family dwellings.