Sammamish, WA Accessory Structures: Garage Conversions (2026)

Sammamish does not publish a standalone garage-conversion rule, but converting an attached or built-in garage into a habitable accessory living unit is treated as creating an attached accessory dwelling unit (ADU). The city's ADU guidance specifically lists ADUs in a basement, above an attached garage, or within a home addition as attached configurations. An attached ADU may be up to 50% of the size of the single-family residence or 1,000 square feet, whichever is larger, and ADUs do not count toward the home's Floor Area Ratio. The city cites ADU standards under SMC 21.05.010.C (footnote 5). On parking, no additional parking is required when the property already has four or more parking spaces (garage plus driveway) - relevant because converting a garage removes covered parking. Any conversion requires building permits, and electrical and plumbing work needs separate permits. Washington's HB 1337 (RCW 36.70A.681) reinforces this permissive approach by requiring cities to allow ADUs at a minimum size of 1,000 square feet, barring owner-occupancy mandates, and prohibiting off-street parking requirements within one-half mile of a major transit stop. Because converting a garage involves egress, light, ventilation, and fire-separation code, confirm requirements with the Sammamish Permit Center before starting.