WA HB1010

Counties may authorize detached accessory dwelling units (ADUs) on rural lots of any size, limited to one ADU per parcel with a maximum floor area of 1,296 square feet

Rural ADUs must meet specific water requirements including using water from the primary dwelling's water right, metering all withdrawals, and cannot be located on parcels using water sources closed to further appropriation

Septic/sewage systems must be inspected by a licensed contractor before building permit issuance to verify capacity for the additional dwelling unit

ADUs must use the same driveway as the primary dwelling, match exterior design of the main home (unless mobile/manufactured), and cannot encroach on critical area buffers or agricultural/forest land

Population growth from compliant rural ADUs cannot be counted toward rural or urban growth targets in comprehensive plans, and counties authorizing rural ADUs must update their comprehensive plans at the next scheduled review