Using Unit Lot Subdivisions to Increase the Local Housing Supply

Editor's note: In 2025, the state legislature passed SB 5559, which supersedes the 2023 law reported on below. The new bill should be followed as guidance and has been summarized in a blog published on June 16, 2025: 2025 Legislation Addressing Residential Unit Lot Subdivisions and Lot Splitting.

In 2023, the Washington State Legislature passed SB 5258 with the purpose of “increasing the supply and affordability of condominium units and townhouses as an option for homeownership.” Among other things, the bill amended the state subdivision statute Chapter 58.17 RCW, requiring that local governments incorporate provisions related to “unit lot subdivisions” into their short plat regulations.

New subsection RCW 58.17.060(3) provides:

All cities, towns, and counties shall include in their short plat regulations procedures for unit lot subdivisions allowing division of a parent lot into separately owned unit lots. Portions of the parent lot not subdivided for individual unit lots shall be owned in common by the owners of the individual unit lots, or by a homeowners' association comprised of the owners of the individual unit lots.

This blog will go into more detail about unit lot subdivisions and provide examples of local regulations allowing their use.

What Are Unit Lot Subdivisions?

Unit lot subdivisions are a method of dividing a single parent parcel into individual “unit lots” that may be sold to individual owners. In contrast, a traditional subdivision or short plat process creates lots that are of “sufficient area and dimension to meet minimum zoning requirements for width and area.” See RCW 58.17.020(9).

A lot created through a traditional subdivision process would be considered the “parent parcel” in the context of a unit lot subdivision. In a unit lot subdivision, the parent parcel must conform to site development and dimensional standards while individual unit lots do not. Any portion of the parent lot that is not part of an individual unit lot is owned in common by the owners of the unit lots.

A unit lot subdivision could be utilized, for example, where middle housing (e.g., duplex, triplex, townhomes, cottage housing) development is allowed, as well as where an accessory dwelling unit is developed on a lot with an existing single-family residence. The footprint containing one individual housing unit (e.g., one townhouse or a third of a triplex) and any associated private open space would be a unit lot.

Unit lot subdivisions are not intended to permit land uses or densities that are not otherwise allowed in the zone in which a unit lot subdivision is proposed. Instead, it is a method of establishing individual ownership of units. Landowners and developers can utilize the unit lot subdivision process to enable separate ownership of units within a parent lot rather than going through the condominium process at Chapter 64.90 RCW.

State Guidance on Unit Lot Subdivisions

The Washington State Department of Commerce (Commerce) has developed a Unit Lot Subdivisions Fact Sheet (Public Draft – Nov. 2024) with helpful information and recommendations for jurisdictions to consider in adopting code that implements RCW 58.17.060(3). The guidance includes:

• graphic representations of different unit lot layouts;
• recommendations for code development, including a model code;
• a table comparing unit lot subdivisions and condominiums; and
• several examples of adopted unit lot subdivision regulations.

Examples of Unit Lot Subdivision Regulations

Many jurisdictions have already adopted unit lot subdivision regulations, and many of these regulations preceded the adoption of SB 5258 in 2023. Some jurisdictions apply different review procedures depending on the number of unit lots created (akin to the conventional short and long subdivision processes).

• Renton Municipal Code Sec. 4.7.090 — Requires nine-unit lots or fewer to be processed as a short plat; 10-unit lots or more are processed as a subdivision.
• Snohomish Municipal Code Sec. 14.215.125 — Requires four-unit lots or fewer to be processed as a short plat; five-unit lots or more are processed as a subdivision.
• Spokane Municipal Code Sec. 17G.080.065 — Requires nine-unit lots or fewer to be processed as a short plat; 10-unit lots or more are processed as a subdivision.
• Wenatchee Municipal Code Sec. 11.32.080 — Nine-unit lots or fewer require administrative review; 10 unit lots or greater require quasi-judicial review.

Here are some examples of codes that do not include different review processes depending on the number of unit lots created:

• Bellevue Municipal Code Sec. 20.45A.065 — Platting and Subdivisions - Special requirements for unit lot subdivisions
• Lake Stevens Municipal Code Sec. 14.18.320 — Subdivisions, Boundary Line Adjustments and Binding Site Plans - Unit lot subdivisions
• Snohomish County Code Sec. 30.41A.205 — Subdivisions - Design standards - Unit lot subdivision

Timeline for Compliance

According to Commerce’s Unit Lot Subdivisions Fact Sheet, jurisdictions must incorporate unit lot subdivision regulations into their short plat regulations by their next periodic update of their comprehensive plan and development regulations as set forth in the Growth Management Act (GMA) at RCW 36.70A.130. Note, however, that the requirement to amend short plat regulations to include unit lot subdivisions applies to all cities and counties throughout the state, not just GMA-planning jurisdictions. Further, the compliance deadline language in SB 5258 is within a section of the bill amending RCW 82.02.060 related to impact fees. It does not refer to the section of the bill amending Chapter 58.17 RCW.

So, while the provision at RCW 58.17.060(3) technically became effective on July 23, 2023, there is not a clear compliance deadline for local governments. As noted above, several jurisdictions have already adopted unit lot subdivision regulations, although many others have not. MRSC recommends that cities and counties work with their attorneys in developing and adopting code provisions as soon as practical.

Additional Resources

For more information, see the following resources:

• Master Builders Association of King and Snohomish Counties: Housing Toolkit
• MRSC: Subdivisions
• Tacoma: Unit Lot Subdivision Summary Sheet