Seattle’s DADU (Detached Accessory Dwelling Unit) regulations have evolved significantly over the past several years. Here is a complete, current guide to what you can build, where, and what the permit process looks like in 2026.
What Is a DADU?
A DADU (Detached Accessory Dwelling Unit) is a separate, self-contained dwelling unit built on the same lot as a single-family home — commonly called a backyard cottage. Seattle also allows one attached ADU (basement unit or added suite) per lot, and in most zones you can have both.
Seattle DADU Size and Height Limits (2026)
- Maximum size: 1,000 sq ft or 60% of the primary dwelling floor area, whichever is greater
- Maximum height: 24 feet (with pitched roof) or 18 feet (flat roof)
- Minimum size: No minimum size specified
Setback Requirements
- Rear setback: 5 feet minimum
- Side setback: 5 feet minimum
- Front setback: DADUs must be behind the primary structure or at least 20 feet from the front lot line
Lot Coverage
Adding a DADU counts toward your lot coverage limit (typically 35% in SF zones). Check your specific parcel on the Seattle GIS portal before designing.
Owner Occupancy — No Longer Required
Since 2019, Seattle no longer requires the property owner to occupy one of the units on a lot with an ADU. Both the primary home and the DADU can be rented simultaneously.
Parking
No additional off-street parking is required for a DADU if the lot is within a quarter mile of a frequent transit stop. Most Seattle lots qualify.
Permit Process
DADUs require a full building permit from SDCI including site plan, architectural drawings, structural drawings, and energy calculations. Current SDCI review time: 8-14 weeks for standard review. See our full guide: Seattle ADU Permit Timeline 2026.
Piper Cole Architects designs and permits DADUs throughout Seattle. See our ADU service or contact us for a free consultation. Call 425-753-6452.
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Piper Cole Architects offers a free initial consultation for all project types — residential, commercial, ADU, and renovation. No obligation. Based in Kirkland, WA. Serving the entire Seattle metro area since 2000.