Ask the Expert - What is an Accessory Dwelling Unit?

  • by Snohomish County Camano Association of REALTORS®
  • Dec 18, 2023, 16:28 PM

Q: What is an Accessory Dwelling Unit?

A: According to the Municipal Research and Services Center of Washington (MSRC), an accessory dwelling unit (or ADU) is a self-contained residential unit located on the same lot as an existing single-family home. Colloquially called “mother-in-law apartments” or “in-law suites” due to the prominence of extended family usage, ADUs have become an increasingly attractive concept for renters and homeowners of differing backgrounds. For many reasons, ADUs have become the subject of much interest to REALTORS®, homeowners, and renters alike.

One of the major benefits of ADUs is their versatility. ADUs come in various forms, catering to different housing needs. Detached ADUs (DADUs), standalone structures separated from the primary dwelling, offer both privacy and independence for residents. Attached ADUs seamlessly integrate with the principal residence, providing a convenient expansion of living space. Interior conversion ADUs – dwellings repurposed from existing spaces such as garages, basements, and attics – offer a budget-friendly alternative to new construction. This diversity in design makes ADUs adaptable to various property layouts and homeowner preferences.

While the history of ADUs can be traced back to centuries-old practices of extended families living together, it wasn't until the last few decades that municipalities across the globe began recognizing the potential of ADUs as a response to housing shortages and multigenerational family structures. The rise of ADUs can be attributed to their flexibility, cost-effectiveness, and ability to adapt to changing demographics.

In recent years, ADUs have gained significant traction as a solution to address housing shortages, increase property value, and promote sustainable living. As cities grapple with housing challenges, ADUs emerge as a versatile and practical option that caters to diverse housing needs. Cities like Seattle and Portland have adopted policies to promote ADU development as part of a strategy to increase middle housing opportunities.

While most cities limit ADU development through zoning rules and strict building regulations, in 2023, the Washington State legislature took a massive step towards freeing developers and homeowners to build such dwellings on their properties. Through one piece of legislation, HB 1337, the legislature moved towards making ADUs more accessible. Changes brought about by this bill include: easing the restrictions on rural zoning, prohibiting excessive impact fees, and increasing the number of ADUs permitted on a parcel of land. While these are just some of the changes, the signing of HB 1337 signals a dramatic push towards increased access to developing – and residing in – ADUs.

However, while there are admirable measures being taken towards increasing the availability of ADUs in urban areas, rural ADU development has been curtailed by many local legislative bodies. In efforts to reduce urban sprawl, many municipalities have become exceedingly cautious about the extent to which they allow development to occur in rural spaces. Recently, Snohomish County legislation which allowed for greater use of ADUs in rural parts of the county was walked-back due to push back from Futurewise, a non-profit advocacy firm focused on planning and land-use policies.

Despite pushback, ADUs offer many benefits. They contribute to increased housing density within established neighborhoods, offering an affordable housing option. Multigenerational living is facilitated, allowing families to stay connected while maintaining individual privacy. Homeowners can leverage ADUs for additional income by renting them out, offsetting mortgage costs or providing supplementary financial support. Moreover, the addition of an ADU often leads to a significant enhancement of the overall property value, making it a valuable investment.

When asked about ADU development in Snohomish County, SCCAR President Julie Love responded that, “ADUs are a valuable option for homeowners. They not only increase property values, but they also add much needed housing options for residents who are seeking a place to live.”

Nevertheless, challenges to ADUs still exist. Zoning regulations, building codes, and neighborhood concerns often present hurdles for both legislators seeking to amend the city codes and homeowners seeking to add an ADU to their property. Advocacy for revised zoning regulations and community engagement are crucial in overcoming these challenges. Additionally, promoting more widespread financing options and incentives can help make ADU construction financially viable for homeowners. Education and open dialogue within communities are vital to address concerns related to privacy, parking, and neighborhood aesthetics, fostering a collaborative approach to ADU integration.

As the world continues to grapple with housing shortages and changing demographics, Accessory Dwelling Units propose an attractive solution that aligns with the principles of sustainable urban development. By providing flexible housing options, fostering multigenerational living, and by contributing to community growth and well-being, ADUs have the potential to reshape the housing market in Snohomish County. As cities grow and change, embracing ADUs could be a key component of building more inclusive, dynamic, and resilient communities one dwelling unit at a time.

For more information about buying or selling a home, be sure to work with a REALTOR®, a member of the National Association of REALTORS®. Snohomish County-Camano Association of REALTORS® is the voice for real estate in Snohomish County. If you have questions for The Expert about real estate email us at, or by visit