This bill creates a new section in chapter 36.70A RCW and amends RCW 36.70A.696 to authorize counties that plan under RCW 36.70A.040 to allow detached accessory dwelling units (ADUs) outside urban growth areas if the county adopts regulations meeting specified requirements. It is a statutory change that permits detached ADUs in rural areas subject to development limitations and procedural obligations for counties. The bill sets substantive development limits and requirements for detached ADUs: one attached or detached ADU per parcel; detached ADUs must meet state and county water supply and any Department of Ecology groundwater mitigation requirements; applicants must document sewage/septic capacity; gross floor area is limited to the size allowable for an attached ADU expansion and to no more than 1,296 square feet (excluding garages, porches, unfinished basements); detached ADUs must use the same driveway and be sited within 150 feet of the principal unit; parcels may not be subdivided to evade limits; combined water withdrawals must meet domestic use limits and each dwelling’s withdrawal must be metered; detached ADUs may not be built on land designated as forestland; and if a detached ADU is offered as a short-term rental, the primary unit must be owner-occupied. The bill also imposes procedural and planning requirements on counties that allow detached ADUs: annual tracking and reporting to “the department” of completed detached ADU permits and detached ADUs used as short-term rentals; required updates to county comprehensive plans at the next and subsequent required reviews to account for completed and projected detached ADUs over a 20-year planning period; and a limit on future comprehensive plan amendments that adjust rural density for actual and projected detached ADUs to no more often than once every five years. The new subsection is expressly stated to be in addition to other county authority and not to invalidate preexisting county ADU ordinances. The text provided does not identify which state department is the reporting recipient, does not specify the new section number or the section’s effective date, and does not include enforcement mechanisms, penalties, or administrative procedures for compliance or noncompliance.

If enacted, counties that choose to allow detached accessory dwelling units (ADUs) outside urban growth areas would create a new, limited pathway for homeowners to add separate rental or living units on the same lot, subject to strict rules: only one ADU per parcel, a maximum size generally capped at 1,296 square feet, must be within 150 feet and share the principal unit’s driveway, meet water supply and septic capacity rules, be metered for water use, and cannot be placed on designated forestland. Homeowners who want to use a detached ADU as a short-term rental would face an owner-occupancy requirement for the main home, and parcels cannot be subdivided to get around these limits. Counties that adopt this option will take on new recurring responsibilities and likely modest administrative costs: they must track and annually report completed detached ADU permits and short-term rental use to an unnamed state department, and they must update their comprehensive plans using the collected data to project 20-year development and limit certain plan amendments to once every five years. The bill leaves key details unclear for implementation—most importantly which state department receives the reports, the section’s effective date, and how compliance will be enforced—so counties and property owners will face uncertainty until those items are clarified.