1. (1) Except as provided in RCW 36.70A.635(4) and 36.70A.636(3), any city that is required or chooses to plan under RCW 36.70A.040 must authorize by ordinance and incorporate into its development regulations, zoning regulations, and other official controls, the following:

   1. (a) Cities with a population of at least 75,000, based on office of financial management population estimates must, on all lots zoned predominantly for residential use, permit the development of:

      1. (i) At least four units per lot, unless zoning permitting higher densities or intensities applies;
      2. (ii) At least six units per lot if located within a quarter-mile walking distance of a major transit stop, unless zoning permitting higher densities or intensities applies; and
      3. (iii) At least six units per lot, if at least two of the units are affordable housing, unless zoning permitting higher densities or intensities applies.

   2. (b) Cities with a population less than 75,000 but at least 25,000 based on office of financial management population estimates must, on all lots zoned predominantly for residential use, permit the development of:

      1. (i) At least two units per lot, unless zoning permitting higher densities or intensities applies;
      2. (ii) At least four units per lot if located within a quarter-mile walking distance of a major transit stop unless zoning permitting higher densities or intensities applies; and
      3. (iii) At least four units per lot, if at least one of the units is affordable housing unless zoning permitting higher densities or intensities applies.
2. (2) Cities with populations under 25,000 based on office of financial management population estimates and within a contiguous urban growth area with the largest city in a county with a population of more than 275,000 must permit the development of at least two units on all lots zoned predominantly for residential use, unless zoning permitting higher densities or intensities applies.
3. (3) Cities are not required to achieve the per unit density on lots after subdivision below 1,000 square feet unless the city chooses to enact smaller allowable lot sizes.

4. (4) (a) To qualify for the additional affordable housing units allowed under subsection (1) of this section, the applicant must:

   1. (i) Commit to renting or selling the required number of units as affordable housing and maintain the units as affordable for a term of at least 50 years; and
   2. (ii) Have the property satisfy that commitment and all required affordability and income eligibility conditions adopted by the local government under chapter 36.70A RCW.

   3. (b) A city must require the applicant to record a covenant or deed restriction that:

      1. (i) Ensures the continuing rental of units subject to these affordability requirements consistent with the conditions in chapter 84.14 RCW for a period of no less than 50 years; and
      2. (ii) Addresses criteria and policies to maintain public benefit if the property is converted to a use other than which continues to provide for permanently affordable housing.

   4. (c) The units dedicated as affordable must:

      1. (i) Be provided in a range of sizes comparable to other units in the development;
      2. (ii) To the extent practicable, have the number of bedrooms in the same proportion as the number of bedrooms in units within the entire development;
      3. (iii) Generally be distributed throughout the development and have substantially the same functionality as the other units in the development.
   5. (d) For cities that have enacted a program under RCW 36.70A.540, the terms of that program govern to the extent they vary from the requirements of this subsection.
5. (5) A city that had enacted a program under RCW 36.70A.540 may require any development, including development described in subsection (1) of this section, to provide affordable housing, either on-site or through an in-lieu payment. The city may expand such a program or modify its requirements.
6. (6) (a) As an alternative to the density requirements in subsection (1) of this section, a city may implement the density requirements in subsection (1) of this section for at least 75 percent of lots in the city that are primarily dedicated to single-family detached housing units.

7. (b) The 25 percent of lots for which the requirements of subsection (1) of this section are not implemented must include, but are not limited to:

   1. (i) Any areas within the city for which the department has certified an extension of the implementation timelines under RCW 36.70A.637 due to the risk of displacement;
   2. (ii) Any areas within the city for which the department has certified an extension of the implementation timelines under RCW 36.70A.638 due to a lack of infrastructure capacity;
   3. (iii) Any lots, parcels, and tracts designated with critical areas or their buffers that are exempt from the density requirements as provided in RCW 36.70A.635 (8)(a). In making this exclusion, only lots which cannot reasonably be developed for middle housing due to the presence of critical areas or their buffers should be included;
   4. (iv) Any portion of a city within a one-mile radius of a commercial airport with at least 9,000,000 annual enplanements that is exempt from the parking requirements under RCW 36.70A.635 (7)(b); and
   5. (v) Any areas subject to sea level rise, increased flooding, susceptible to wildfires, or geological hazards over the next 100 years.

8. (c) Unless identified as at higher risk of displacement under RCW 36.70A.070 (2)(g), the 25 percent of lots for which the requirements of subsection (1) of this section are not implemented may not include any areas:

   1. (i) For which the exclusion would further racially disparate impacts or result in zoning with a discriminatory effect;
   2. (ii) Within one-half mile walking distance of a major transit stop; or
   3. (iii) Historically covered by a covenant or deed restriction excluding racial minorities from owning property or living in the area, as known to the city at the time of each comprehensive plan update.
9. (d) Cities may make an application to the department to include more than 25 percent of lots for which the requirements of subsection (1) of this section do not apply, subject to the certification process provided for in chapter 365-199 WAC.

10. (7) Cities subject to the requirements of subsection (1)(a) or (b) of this section must allow at least six of the nine types of middle housing to achieve the unit density required in subsection (1) of this section. Cities subject to the requirements of subsection (2) of this section should allow at least as many middle housing types that can be developed as two unit per lot projects.

    1. (a) Cities may allow accessory dwelling units to achieve the unit density required in subsection (1) of this section.
    2. (b) Cities are not required to allow accessory dwelling units or middle housing types beyond the density requirements in subsection (1) of this section.
    3. (c) Cities must allow zero lot line short subdivisions where the number of lots created is equal to the unit density required in subsection (1) of this section.
    4. (d) Single family detached dwellings are not a middle housing type and may not count toward the unit density requirements of RCW 36.70A.635(1).
11. (8) (a) Cities shall not require through development regulations any standards for middle housing that are more restrictive than those required for detached single-family residences.
12. (b) Cities may apply any objective development regulations that are required for detached single-family residences including, but not limited to, setback, lot coverage, stormwater, clearing, and tree canopy and retention requirements.

13. (c) Cities may apply design review for middle housing provided:

    1. (i) Only administrative design review shall be applied; and
    2. (ii) Only those objective design standards necessary to address middle housing compatibility with the scale, form, and character with single-family houses are applied.
    3. (9) Cities shall apply to middle housing the same development permit and environmental review processes that apply to detached single-family residences, unless otherwise required by state law including, but not limited to, shoreline regulations under chapter 90.58 RCW, building codes under chapter 19.27 RCW, energy codes under chapter 19.27A RCW, or electrical codes under chapter 19.28 RCW.

    4. (10) The provisions of this section do not apply to:

       1. (a) Portions of a lot, parcel, or tract with designated critical areas under RCW 36.70A.170 or their buffers as required by RCW 36.70A.170, except for critical aquifer recharge areas where a single-family detached house is an allowed use provided that any requirements to maintain aquifer recharge areas are met;
       2. (b) Areas designated as sole-source aquifers by the United States Environmental Protection Agency on islands in the Puget Sound;
       3. (c) 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));
       4. (d) Lots designated urban separators by countywide planning policies as of July 23, 2023; or
       5. (e) A lot that was created through the splitting of a single residential lot.

    5. (11) RCW 36.70A.635 does not:

       1. (a) Prohibit a city from permitting detached single-family residences;
       2. (b) Require a city to issue a building permit if other federal, state, and local requirements for a building permit are not met.

    6. (12) A city must comply with the requirements of this section on the latter of:

       1. (a) Six months after its next periodic comprehensive plan update required under RCW 36.70A.130 if the city meets the population threshold based on the 2020 office of financial management population data; or
       2. (b) Twelve months after their next implementation progress report required under RCW 36.70A.130 after a determination by the office of financial management that the city has reached a population threshold established under this section.
    7. (13) A city complying with this section and not granted a timeline extension under RCW 36.70A.638 does not have to update its capital facilities plan element required by RCW 36.70A.070(3) to accommodate the increased housing required in RCW 36.70A.635 or 36.70A.636 until the first periodic comprehensive plan update required for the city under RCW 36.70A.130(5) that occurs on or after June 30, 2034.
    8. (14) Until June 30, 2026, for cities subject to a growth allocation adopted under RCW 36.70A.210 that limits the maximum residential capacity of the jurisdiction, any additional residential capacity created by this section for lots, parcels, and tracts outside of critical areas or their buffers may not be considered an inconsistency with the countywide planning policies, multicounty planning policies, or growth allocations adopted under RCW 36.70A.210.

    9. (15) Recommendations for meeting requirements.

       1. (a) Cities should define "all lots zoned predominantly for residential use" with consideration given to:

          1. (i) Including zoning districts where residential dwellings are the primary use;
          2. (ii) Nonresidential zones, such as commercial, industrial, and public zoning districts, should not be considered lots "zoned predominantly for residential use" even though they may permit single-family dwellings;
          3. (iii) Mixed use zones that allow for a complementary mix of commercial development with residential development, and which allow residential development at higher densities than middle housing, should not be considered lots predominantly zoned for residential use.
       2. (b) Cities may define duplex, triplex, fourplex, fiveplex, and sixplex provided that the definitions are consistent with the definition of middle housing in RCW 36.70A.030(26), including that middle housing buildings are compatible in scale, form, and character with single-family houses and contain two or more attached, stacked, or clustered homes.

    10. (16) Development regulations for middle housing:

        1. (a) Shall not require any standards that are more restrictive than those required for detached single-family residences, except as provided for in RCW 36.70A.635 (6)(a) through administrative design review;
        2. (b) May apply any objective development regulations that are required for detached single-family residences including, but not limited to, set-back, lot coverage, stormwater, clearing, and tree canopy and retention requirements;
        3. (c) May adopt objective development regulations for middle housing that are less restrictive than existing standards required for detached single-family residences; and

        4. (d) May use administrative design review to adopt design and development standards that reflect differences between detached single-family residences and "middle housing" types, provided that:

           1. (i) The design and development standard is objective; and
           2. (ii) The design and development standard makes middle housing compatible with the form, character, and scale of existing single-family houses.

        5. (e) Cities establishing unit per lot requirements above the minimums identified in RCW 36.70A.635 (1)(a)(i) through (iii), (b)(i) through (iii) and (c), should consider:

           1. (i) The variety of lot sizes that may exist in the city;
           2. (ii) Proximity to major transit facilities, if any;
           3. (iii) The type of major transit facilities, if any;
           4. (iv) Neighborhood facilities, such as shopping services, if any;
           5. (v) Existing public facilities such as sidewalks;
           6. (vi) How objective middle housing development and design standards can serve to make middle housing compatible with the form, scale, and character of single family homes.
        6. (f) Cities must apply the same critical area requirements for middle housing development that would apply to single family homes on the same lot unless an analysis, including best available science, shows that more restrictive standards are necessary to protect critical area functions and values.
    11. (17) A city complying with the requirements of RCW 36.70A.635 and not granted a timeline extension under RCW 36.70A.638 should update its capital facilities element to accommodate the increased housing required by RCW 36.70A.635 and 36.70A.636 prior to the first periodic update that occurs on or after June 30, 2034.

[Statutory Authority: RCW 36.70A.050 and 36.70A.190. WSR 25-17-058, s 365-196-890, filed 8/15/25, effective 9/15/25.]