Seattle DADU Rules in 2026: What You Can Actually Build

Detached accessory dwelling units — backyard cottages — are now one of the most accessible ways to add housing and rental income on a Seattle lot. Here is what the rules look like in 2026, in plain language.

How many units can you build?

Under Washington's HB 1337, cities in urban growth areas — including Seattle, Bellevue, Kirkland, and Redmond — must allow at least two accessory dwelling units per residential lot. In practice, that commonly means one attached ADU plus one DADU, or two DADUs, alongside your main house. Exact combinations depend on your city and lot.

Key Seattle DADU rules to know

- Size: DADUs in Seattle are generally limited to around 1,000 square feet of living space.

- Owner occupancy: Seattle removed its owner-occupancy requirement, so you can rent out both your main house and your DADU long-term.

- Parking: no additional off-street parking is required for an ADU in Seattle.

- Height and setbacks: limits vary by zone and lot; a design-build contractor confirms these before design begins.

Rules differ city by city — Bellevue, Kirkland, and unincorporated King County each have their own specifics, which is why we verify zoning and utilities on every project before design starts.

What a DADU costs in 2026

Most detached ADUs in the Seattle–Bellevue area run $250,000–$450,000+ depending on size, site access, utility connections, and finishes. A garage conversion can come in lower. Well-built DADUs near Seattle commonly rent long-term for $2,000–$3,200+ per month.