Cowlitz County, WA Accessory Structures: ADU Rules (2026)

585, part of the Land Use Ordinance, expressly promotes ADUs as a way to give property owners housing options for families and rental income. An ADU is allowed on a lot occupied by an existing single-family dwelling in every Comprehensive Plan designation and every zoned district except the manufactured home (MH), light manufacturing (ML), and commercial C-1 and C-2 districts. The parcel must be at least 6,000 square feet, and the ADU can be built within, attached to, or fully detached from the primary house.

05 CCC. 10 CCC's dimensional standards for that zone. In areas classified urban or suburban in the Comprehensive Plan, a lot may host one attached ADU and one detached ADU simultaneously; everywhere else, only a single ADU per lot is allowed. Attached units need at least one external entry, and the county requires one additional on-site parking space per ADU while barring new curb cuts to serve it, so parking has to be accommodated within the existing driveway. The Director may relax standards to add accessibility features.

A prior provision allowing a stricter limit was repealed by Ord. 20-036, and the county separately allows temporary ADUs for medical hardship situations, removable once the hardship ends or the property sells, under a recorded covenant. Building permits from the Building and Planning Department are mandatory to confirm fire, health and safety code compliance before occupancy.