Seattle has made it a lot easier to build ADUs (accessory dwelling units) in recent years. On most single-family lots, you can have two ADUs. One must be attached to your main house (think basement apartment or converted garage) and the other may be a detached structure in your backyard. The city dropped the old rule that required the owner to live on-site, so you can rent out all three units if you want. Detached ADUs can be up to 1,000 square feet, and the height limit is generally 23-25 feet depending on your lot. You'll still need to pull a building permit and meet the usual zoning rules for setbacks, lot coverage, and parking. But compared to a few years ago, the process is way more straightforward. Check Seattle's Department of Construction and Inspections for the latest permit requirements.
What are the ADU requirements in Seattle in 2026?
Seattle allows up to two ADUs on most single-family lots — one attached (like a basement unit) and one detached (like a backyard cottage). There's no owner-occupancy requirement anymore, and detached ADUs can be up to 1,000 square feet. You still need a building permit and have to meet setback and height limits, but the city removed a lot of the old barriers. Homeowners can also sell the ADU's individually by creating a Condo Association.
Key Points
- Seattle permits up to two Accessory Dwelling Units (ADUs) on a single-family lot in various configurations.
- No off-street parking or owner-occupancy is required for properties with ADUs.
- ADUs can be at least 1,000 square feet, with flexible rules for setbacks and converting existing structures like garages.
- Despite permissive ADU rules, other site-specific regulations for critical areas like steep slopes may still apply.
Full Legal Analysis
Of course, I can help you with that. Here are the key requirements for building an Accessory Dwelling Unit (ADU) in Seattle.
In Seattle, you can generally build up to two ADUs on a lot that has a single-family home. The city’s rules are designed to be flexible, especially regarding size, placement, and parking.
Under Seattle's code, you are allowed to have a maximum of two ADUs on your property [cite: 1]. These can be in a few different configurations: both attached to the main house, both detached in the backyard (either in one or two separate structures), or one of each. You are not required to provide any off-street parking for an ADU. Furthermore, state law prohibits cities from requiring you to live on the property as a condition of having an ADU [cite: 4].
Here are some of the key development rules:
- Size: State law requires cities to allow ADUs of at least 1,000 square feet. In Seattle, if you are creating an attached ADU within a part of your house that existed as of July 23, 2023, it can even exceed 1,000 square feet [cite: 1].
- Setbacks: A detached ADU has no required setback from a property line that abuts an alley. In Neighborhood Residential zones, a detached ADU in the rear yard only needs to be 5 feet from any other lot line.
- Existing Garages: The rules are even more flexible if you want to convert an existing accessory structure, like a garage, into an ADU. The city may waive certain development standards, such as lot coverage or setbacks, to make the conversion possible [cite: 1].
One thing to keep in mind is that while the ADU rules are permissive, other regulations still apply. For example, if your property is in a critical area like a shoreline or on a steep slope, there may be additional restrictions on what you can build [cite: 6].
Because the exact rules can depend on your specific property and its zoning, a good next step is to check with the Seattle Department of Construction and Inspections (SDCI). They can provide guidance tailored to your address.
Need a More Specific Answer?
Search LawAccess for your exact situation with full legal citations.
Search on LawAccess