Accessory Dwelling Units: Overview & Resources
An accessory dwelling unit (ADU) is a is a secondary housing unit on a single-family residential lot. An ADU has all the basic facilities needed for day-to-day living independent of the main home, such as a kitchen, sleeping area, and a bathroom. As the term “accessory” implies, ADUs are generally smaller in size and prominence than the main residence on the lot. These units are not legal unless they have been established through a local permitting process.
A secondary dwelling unit attached to the main home is called an attached accessory dwelling unit (AADU).
A detached accessory dwelling unit (DADU) is a separate structure from the main house. It is typically located in the rear yard of the property or above or within a converted garage. DADUs are often referred to as a backyard cottage.
Types of ADUs
- Detached new construction DADUs
- Garage conversion ADUs
- ADUs above a garage or workshop (sometimes called carriage houses)
- Addition to the main house or bump-out ADUs
- Basement conversion ADUs
- Internal ADUs, where part of the primary house besides the basement is converted to an ADU.
What are the benefits of ADUs?
- Accessory dwelling units also offer a good housing option for various household types, including families with children and multi-generational households.
- Encouraging ADUs can increase the supply and variety of housing options in single-family neighborhoods. They can promote economic diversity in neighborhoods that might be out of range for average-income renters.
- DADUs share many characteristics with small single-family houses: a single unit with no shared walls in quiet residential neighborhoods. Due to their smaller size and lack of additional land cost, backyard cottages can offer a more affordable housing option in neighborhoods where homes are unaffordable for many people.
- Adding an ADU can allow families to respond to changing needs for living space such as multi-generational living or aging in place.
- Property owners can also rent their ADU to earn additional income that makes it easier for them to remain in their neighborhood, help pay the mortgage, or pay for home improvements.
- DADUs are an example of infill development, which slowly increases density and takes advantage of existing infrastructure. Both ADUs and DADUs can help reduce sprawl and preserve the character of neighborhoods.
- ADUS can promote economic diversity in neighborhoods that might be out of range for average-income renters.
- ADUs provide flexible dwelling options in city neighborhoods utilize existing governmental infrastructure (roads, sewers, schools), and reduce the demand for expanding infrastructure in far-lying reaches of a developed metropolitan area.
- ADUs provide housing with a relatively small environmental footprint.
Accessory Dwelling Units (A Regional Coalition for Housing)
How to Add an In-Law Suite to Your Home (This Old House, 4/18/23)
Washington Takes a Stand for Granny Flats (Sightline, 3/11/20)
Seattle City Council votes to reduce barriers to building ADUs (Curbed – 7/1/19)
Seattle Says Yes to the Best Rules in America for Backyard Cottages (Sightline, 7/1/19)
A large range of municipal land use and zoning regulations differentiate ADU types and styles and dramatically affect their allowed uses. It is important to verify this information with your local building authority. Below are links to ADU regulations in some northwest Washington cities. If your city is not listed, contact your city’s planning or building department.