ADU/DADU - A Guide for Seattle & Surrounding Areas

September 8, 2018 | By Reece Homes Team

One of the more frequently asked questions we received from homebuyers and homeowners in today's market relates to Accessory Dwelling Units (ADU) or Detached Accessory Dwelling Units (DADU). Rising home values and changing demographics are making these opportunities for sharing space much more attractive than they've ever been before. Whether you're looking to offset your monthly housing cost through short or long-term rental, or host a family member/friend/nanny, an accessory dwelling is a great resource for many.

To help you learn more about ADU's and DADU's, we're going to explore the three major questions everyone must ask before they invest significant time or money in an accessory dwelling by looking at the answers provided by the city of Seattle, King County and Snohomish County. While all communities with zoning regulations will have slightly different regulations and requirements, these three municipalities provide a great cross-section to get started and learn more. If you're interested in the regulations for your specific community, please give us a call or send us an email and we'll be happy to help you find the right resources and get answers.


What specifically is an accessory dwelling unit in my community?

We will start here to ensure your definition of an accessory dwelling matches your local community definition since if there are major differences you may not be able to create an accessory dwelling meeting your needs or face a difficult process of land use variance or conditional use permit.


Seattle: "An accessory dwelling unit or detached accessory dwelling unit (sometimes called a mother-in-law apartment) is a separate living space within a house or on the same property as an existing house. These units aren’t legal unless they have been established through a permit process..."

King County: Their definition is a little less helpful as it does not provide the clear definition of other communities… The best clarification we could find came from the FAQ on the relevant question within the permits website of the County: "ADUs differ from accessory living quarters in that an ADU is allowed to have a kitchen or kitchen facilities as defined under KCC 21A.06.0662 "

Snohomish County: Here they are very clearly defined… ""Accessory apartment-attached" means a dwelling unit that is in the same structure as, under the same ownership as, and subordinate to an owner-occupied single-family dwelling unit..." / ""Accessory apartment-detached" means a dwelling unit other than a mobile home, which is located on the same lot and under the same ownership as, and subordinate to, an owner-occupied single-family dwelling unit, except that..."


Is an accessory dwelling unit allowed in my community?
While many communities allow accessory dwellings, they may not allow them in all land use zones or even all residential zones. It's important to know what zone your property is in and compare it to the allowed zones for accessory dwellings in your community. You can determine your zoning by looking up your property on your County website (King County, Snohomish County), or by giving us a call.

Seattle: They are allowed in nearly all single-family and low rise zones in the city of Seattle.

King County: According to King County code section 21A .08.030 "residential accessory uses" are allowed in all zones except Mineral and Industrial within the county. "Accessory living quarters and dwellings" (21A.06.020) are included in the definition of "residential accessory uses".

Snohomish County: Accessory apartments are allowed in nearly all single-family and multifamily residential urban zones in Snohomish County as a "Administrative Conditional Use" which requires a review and permit by the County to ensure the use complies with code and regulations, though may not require public hearing.


What are the regulations and requirements for accessory dwelling units in my community?

Like all structures and dwellings, accessory dwellings have many requirements from communities related to their size, lot size/setbacks, building codes and other details which are important to consider prior to investing time and money in the next steps of discovery and planning.

Seattle: Owner is required to live in the primary or accessory unit, the size and placement are limited by code and further restrictions include lot size, parking, building codes and more. There's a great website with plenty of information and links to everything you need for discovery, planning and even permitting.


King County: Like other communities, King County has detailed regulations which typically require owner occupancy of the primary or accessory dwelling, a single accessory dwelling on a property, and compliance with regulations on unit size/lot size/parking/etc. as well as building to current codes. We suggest beginning your review of these details in section 21A.08.30.B.7a, though acknowledge there are likely plenty more regulations you'll want to discuss directly with County planners should you pursue an ADU or DADU.

Snohomish County: Once again, Snohomish County has very clear definitions…  In most cases, limited to a single accessory dwelling on a property, requirement for owner occupancy in the primary or accessory dwelling, compliance with current building codes and specific regulations on size/lot size/parking/etc. The full details begin with section 30.28.010 in Snohomish County code.


Next Steps

If you are still reading, chances are an accessory dwelling may be in your future. We'd love to answer your questions and help you with your next steps whether it's a visit to your local planning and development office, talking with an architect or experienced general contractor or a detailed discussion of the costs, benefits and drawbacks of adding an accessory dwelling to your property. Feel free to give us a call or send us an email so we can answer your questions and help you plot your next steps.



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