HIP Tool: Accessory Dwelling Units (ADU)

An accessory dwelling unit (ADU) is a small, self-contained residential unit built on the same lot as an existing single family home. ADUs may be built within a primary residence (e.g., basement unit) or detached from the primary residence. They can be an effective way to add variety and affordable rental housing stock to existing single family neighborhoods.


What issues do accessory dwelling units address?

Housing Diversity. Accessory dwelling units add variety and housing choice in single family neighborhoods. Units are generally smaller than traditional single family homes. In addition to adding different sizes and forms of housing, ADUs can also add rental opportunities to largely owner-occupied neighborhoods. ADUs can be a great option for allowing residents to age in place or live with or near family and caregivers, providing a flexible way to address family needs for additional housing.

Affordability. In expensive single family dominated areas, accessory dwelling units can also provide affordable housing choices. Most communities require an ADU to be smaller than the primary home on the property, and the smaller size can reduce the rental price of the unit. Monthly rent of the unit would likely be lower than a mortgage payment for a house in the same neighborhood. Residents would likely not bear the maintenance and other costs associated with owning a home. Depending on how the ADU is constructed, residents may be able to share utility costs with the primary residence. For example, if the unit is attached to the primary residence, utility costs may be lowered by the simple efficiency of shared walls. For homeowners, an ADU can be an additional source of income for property owners, offsetting the cost of home ownership.

Density. Accessory dwelling units are a way to create infill housing and add density to single family neighborhoods without compromising the character or design of a community. ADUs can help jurisdictions achieve housing goals by providing density with an alternative approach to apartment complexes.

Where are accessory dwelling units applicable?

Accessory dwelling units work in every size jurisdiction. While Washington cities and towns with populations greater than 20,000 are required to plan for ADUs in single-family zones (RCW 43.63A.215), smaller cities frequently find on their own that ADUs provide an answer to some of their key housing goals at a variety of densities and neighborhood settings. Larger cities that already allow ADUs may be interested in revisiting their ordinance to expand their application. ADUs are particularly helpful in providing new housing options in cities or neighborhoods that are already built out, or where the character is to remain single family in design but with increases to density.

Accessory dwelling units are excellent tools for adding housing choices in centrally located residential zones. Encouraging units in neighborhoods near transit, shopping and other amenities can provide additional affordability and convenience from reduced transportation costs. ADUs are also effective in rural areas, providing people who work in agricultural or isolated areas with opportunities to locate nearer to their job, without having to purchase a home or large tract of land.

Tool Profile

Focus Areas

  • Expensive Housing Markets
  • Innovative Single Family Techniques

Project Types

  • Single Family
  • Rental
  • Market Rate

Affordability Level

  • Less than 80% AMI

Housing Goal

  • Affordability
  • Diversity