(1)(a) Cities must adopt or amend by ordinance, and incorporate into their development regulations, zoning regulations, and other official controls the requirements of subsection (2) of this section for buildings that are zoned for commercial or mixed use no later than six months after its next periodic comprehensive plan update required under RCW 36.70A.130.

(b) The requirements of subsection (2) of this section apply and take effect in any city that has not adopted or amended ordinances, regulations, or other official controls as required under this section by the timeline in (a) of this subsection and supersede, preempt, and invalidate any conflicting local development regulations.

(2) Through ordinances, development regulations, zoning regulations, or other official controls as required under subsection (1) of this section, cities may not:

(a) Impose a restriction on housing unit density that prevents the addition of housing units at a density up to 50 percent more than what is allowed in the underlying zone if constructed entirely within an existing building envelope in a building located within a zone that permits multifamily housing, provided that generally applicable health and safety standards, including but not limited to building code standards and fire and life safety standards, can be met within the building;

(b) Impose parking requirements on the addition of dwelling units or living units added within an existing building, however, cities may require the retention of existing parking that is required to satisfy existing residential parking requirements under local laws and for nonresidential uses that remain after the new units are added;

(c) With the exception of emergency housing and transitional housing uses, impose permitting requirements on the use of an existing building for residential purposes beyond those requirements generally applicable to all residential development within the building's zone;

(d) Impose design standard requirements, including setbacks, lot coverage, and floor area ratio requirements, on the use of an existing building for residential purposes beyond those requirements generally applicable to all residential development within the building's zone;

(e) Impose exterior design or architectural requirements on the residential use of an existing building beyond those necessary for health and safety of the use of the interior of the building or to preserve character-defining streetscapes, unless the building is a designated landmark or is within a historic district established through a local preservation ordinance;

(f) Prohibit the addition of housing units in any specific part of a building except ground floor commercial or retail that is along a major pedestrian corridor as defined by each city, unless the addition of the units would violate applicable building codes or health and safety standards;

(g) Require unchanged portions of an existing building used for residential purposes to meet the current energy code solely because of the addition of new dwelling units within the building, however, if any portion of an existing building is converted to new dwelling units, each of those new units must meet the requirements of the current energy code;

(h) Deny a building permit application for the addition of housing units within an existing building due to nonconformity regarding parking, height, setbacks, elevator size for gurney transport, or modulation, unless the city official with decision-making authority makes written findings that the nonconformity is causing a significant detriment to the surrounding area; or

(i) Require a transportation concurrency study under RCW 36.70A.070 or an environmental study under chapter 43.21C RCW based on the addition of residential units within an existing building.

(3) Nothing in this section requires a city to approve a building permit application for the addition of housing units constructed entirely within an existing building envelope in a building located within a zone that permits multifamily housing in cases in which the building cannot satisfy life safety standards.

(4) For the purpose of this section, "existing building" means a building that received a certificate of occupancy at least three years prior to the permit application to add housing units.