Sammamish, WA Accessory Structures: ADU Rules (2026)

The City of Sammamish allows up to two accessory dwelling units (ADUs) on a lot with a detached single-family residence, in configurations of two attached, two detached, or one of each. A detached ADU may be a maximum of 1,000 square feet, while an attached ADU may be up to 50% of the size of the single-family residence or 1,000 square feet, whichever is larger. ADUs do not count toward the home's Floor Area Ratio. No additional parking is required when the property already has four or more parking spaces, counting garage space and driveway. Detached-ADU development standards vary by zone, with rear and side setbacks as low as 5 feet and a height limit of 24 feet; the city cites these standards under SMC 21.05.010.C (footnote 5). Sammamish's framework reflects Washington's statewide ADU law, HB 1337 / RCW 36.70A.681, which requires planning cities to allow at least two ADUs per single-family lot inside urban growth areas, prohibits owner-occupancy mandates, bars maximum-size caps below 1,000 square feet, forbids height limits under 24 feet, and prohibits off-street-parking requirements within one-half mile of a major transit stop. Always confirm current standards with the Permit Center before building.