(1) In addition to ordinances, development regulations, and other official controls adopted or amended to comply with this section and RCW
36.70A.680, a city or county must comply with all of the following policies:
(a) The city or county may not assess impact fees on the construction of accessory dwelling units that are greater than 50 percent of the impact fees that would be imposed on the principal unit;
(b) The city or county may not require the owner of a lot on which there is an accessory dwelling unit to reside in or occupy the accessory dwelling unit or another housing unit on the same lot;
(c) The city or county must allow at least two accessory dwelling units on all lots that are located in all zoning districts within an urban growth area that allow for single-family homes in the following configurations:
(i) One attached accessory dwelling unit and one detached accessory dwelling unit;
(ii) Two attached accessory dwelling units; or
(iii) Two detached accessory dwelling units, which may be comprised of either one or two detached structures;
(d) The city or county must permit accessory dwelling units in structures detached from the principal unit;
(e) The city or county must allow an accessory dwelling unit on any lot that meets the minimum lot size required for the principal unit;
(f) The city or county may not establish a maximum gross floor area requirement for accessory dwelling units that is less than 1,000 square feet;
(g) The city or county may not establish roof height limits on an accessory dwelling unit of less than 24 feet, unless the height limitation that applies to the principal unit is less than 24 feet, in which case a city or county may not impose roof height limitation on accessory dwelling units that is less than the height limitation that applies to the principal unit;
(h) A city or county may not impose setback requirements, yard coverage limits, tree retention mandates, restrictions on entry door locations, aesthetic requirements, or requirements for design review for accessory dwelling units that are more restrictive than those for principal units;
(i) A city or county must allow detached accessory dwelling units to be sited at a lot line if the lot line abuts a public alley, unless the city or county routinely plows snow on the public alley;
(j) A city or county must allow accessory dwelling units to be converted from existing structures, including but not limited to detached garages, even if they violate current code requirements for setbacks or lot coverage;
(k) A city or county may not prohibit the sale or other conveyance of a condominium unit independently of a principal unit solely on the grounds that the condominium unit was originally built as an accessory dwelling unit; and
(l) A city or county may not require public street improvements as a condition of permitting accessory dwelling units.
(2)(a) A city or county subject to the requirements of this section may not:
(i) Require off-street parking as a condition of permitting development of accessory dwelling units within one-half mile walking distance of a major transit stop;
(ii) Require more than one off-street parking space per unit as a condition of permitting development of accessory dwelling units on lots smaller than 6,000 square feet before any zero lot line subdivisions or lot splits; and
(iii) Require more than two off-street parking spaces per unit as a condition of permitting development of accessory dwelling units on lots greater than 6,000 square feet before any zero lot line subdivisions or lot splits.
(b) The provisions of (a) of this subsection do not apply:
(i) If a local government submits to the department an empirical study prepared by a credentialed transportation or land use planning expert that clearly demonstrates, and the department finds and certifies, that the application of the parking limitations of (a) of this subsection for accessory dwelling units will be significantly less safe for vehicle drivers or passengers, pedestrians, or bicyclists than if the jurisdiction's parking requirements were applied to the same location for the same number of detached houses. The department must develop guidance to assist cities and counties on items to include in the study; or
(ii) To portions of cities within a one mile radius of a commercial airport in Washington with at least 9,000,000 annual enplanements.
(3) When regulating accessory dwelling units, cities and counties may impose a limit of two accessory dwelling units, in addition to the principal unit, on a residential lot of 2,000 square feet or less.
(4) The provisions of this section do not apply to lots designated with critical areas or their buffers as designated in RCW
36.70A.060, or to a watershed serving a reservoir for potable water if that watershed is or was listed, as of July 23, 2023, as impaired or threatened under section 303(d) of the federal clean water act (33 U.S.C. Sec. 1313(d)).
