Washington State ADU and Backyard Cottage Rules (2026)

Washington state has become one of the most ADU-friendly states in the country. A series of state laws passed in recent years has made it significantly easier for homeowners to build accessory dwelling units (ADUs) and backyard cottages on their properties. Seattle in particular has been a leader in ADU development, and surrounding cities have followed suit. Here is what you need to know about the current rules.

State-Level ADU Legislation

Washington state law (HB 1337, effective 2024) requires all cities planning under the Growth Management Act to allow at least two ADUs per lot in residential zones. Cities cannot require the owner to live on the property as a condition of ADU approval. They also cannot require off-street parking for ADUs within a quarter mile of a major transit stop. The law caps impact fees for ADUs and limits the time cities have to review ADU permit applications. These provisions have effectively standardized the minimum ADU allowances across the state.

Seattle ADU Rules

Seattle allows up to two ADUs per single-family lot: one attached (or internal) ADU and one detached ADU (often called a Detached Accessory Dwelling Unit, or DADU). Detached ADUs can be up to 1,000 square feet or 75% of the principal dwelling's floor area, whichever is less. The maximum height for a detached ADU is 23 feet on standard lots. Seattle does not require owner-occupancy or additional parking for ADUs. The city has pre-approved DADU plans available to streamline the permitting process.

Bellevue and Eastside Cities

Bellevue now allows ADUs on all single-family lots and has eliminated owner-occupancy requirements in compliance with state law. Kirkland has similarly updated its code to allow two ADUs per lot. Redmond and Renton have both adopted ADU-friendly ordinances that follow the state framework. The Eastside cities generally allow detached ADUs up to 1,000 square feet with height limits between 18 and 25 feet depending on the city.

Costs and Considerations

Building an ADU in the Seattle metro area typically costs between $150,000 and $350,000 for a new detached unit, depending on size, finishes, and site conditions. Garage conversions and internal ADUs generally cost less, ranging from $80,000 to $200,000. Utility connection fees, which were previously a major barrier, have been capped by state law for ADUs. Many local utility providers now offer reduced connection fees for ADUs. The permitting timeline in Seattle typically runs 8 to 16 weeks for a standard ADU application.

Rental Potential

ADUs in the Seattle market can generate significant rental income. A one-bedroom DADU in Seattle can rent for $1,500 to $2,500 per month depending on location and quality. In Bellevue and other Eastside cities, rents are comparable or higher. Washington state law prohibits cities from restricting ADU rentals to family members, ensuring that homeowners can rent their ADUs to anyone. Short-term rental rules for ADUs vary by city, so check local ordinances if you plan to use platforms like Airbnb.

Getting Started

Before building an ADU, check your city's specific regulations, even though state law sets a baseline. Review your property's zoning, lot coverage limits, and setback requirements. Consider consulting with a designer or architect experienced in ADU construction in your area. Many cities offer pre-application meetings where staff can walk you through the process and identify potential issues early.