Renton, WA Accessory Structures: ADU Rules (2026)

RMC 4-2-110C sets the citywide ADU standards. Up to two ADUs are permitted per legal lot as an accessory use to a principal dwelling, in any combination of attached and detached units, and they may be built within or added onto the primary dwelling. Each unit's maximum gross floor area and maximum building area are both capped at 1,000 square feet, though a conversion unit that occupies an entire floor of the primary dwelling may exceed that if it stays under 1,250 square feet before being reclassified as an attached dwelling.

Parking requires one space per unit, waived entirely for ADUs within a half-mile walk of a Major Transit Stop. Attached and internal ADUs must meet the setback and height standards that apply to primary structures under RMC 4-2-110A or 4-2-110B. Detached ADUs must sit at least 4 feet from any residential structure or be reclassified as attached, follow the underlying zone's front-yard setback, and observe side setbacks of 25 feet in RC/R-1, 5 feet in R-4/R-6/R-8, and 4 feet in R-10/R-14. The rear setback is 5 feet, with at least 25% of the rear yard lineal length left unoccupied when a detached ADU sits within 10 feet of the rear property line; ADUs may be built at the rear lot line where an alley exists.

Detached ADU building height is capped at 24 feet measured to the highest point of a flat roof or highest ridge of a pitched roof. The section notes ADUs also require compliance with building, fire and energy code and utility access requirements independent of the zoning permit.