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.
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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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Frequently Asked Questions
What is a DADU?
A DADU is a detached accessory dwelling unit: a separate, self-contained living space on the same lot as your main house, with its own kitchen, bathroom, and entrance. It is often called a backyard cottage.
How big can a DADU be in Seattle?
Seattle generally allows detached accessory dwelling units up to about 1,000 square feet of gross floor area. Lot-coverage rules and site-specific limits can reduce that, so the buildable size depends on your particular lot.
How tall can a DADU be in Seattle?
Height allowances for detached ADUs typically fall in the range of roughly 18 to 23 feet depending on lot width, roof form, and green building measures. An architect confirms the exact allowance for your lot.
Is owner occupancy required for a Seattle DADU?
No. Seattle removed the owner-occupancy requirement for accessory dwelling units, so you are not required to live on the property in order to build or rent one.
Is off-street parking required for a DADU in Seattle?
No. Seattle does not require additional off-street parking for accessory dwelling units, which makes many smaller and constrained lots feasible.
How many accessory units can I have on one lot?
Many Seattle lots can have two accessory units, typically one attached ADU and one detached ADU, subject to size, height, and lot-coverage limits.