2025 Legislation Addressing Residential Unit Lot Subdivisions and Lot Splitting

Two bills passed during the last legislative session that will impact the local residential land division process — one provides additional guidance regarding local unit lot subdivision regulation (SB 5559), and the other allows streamlined residential lot splitting under certain circumstances (SB 1096).

SB 5559 — Updated Guidance on Unit Lot Subdivisions

In 2023 the legislature adopted SB 5258 which, among other things, 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. The new subsection at RCW 58.17.060(3) previously read:

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.

Beyond this, the 2023 legislation offered no guidance or details on how local unit lot subdivision regulations should be processed or implemented. In February, MRSC published Using Unit Lot Subdivisions to Increase the Local Housing Supply, which offered an overview of SB 5258, highlighting some local examples and summarizing guidance by the Washington State Department of Commerce.

In adopting SB 5559 this year, the legislature has now offered significantly more guidance and clarification to local governments — including limiting which jurisdictions must include such provisions. The legislation also adds several specific requirements that must be incorporated into local unit lot subdivision regulations. Below are some highlights.

Fewer jurisdictions must comply

Only cities and towns located in counties that are planning under the Growth Management Act (GMA), RCW 36.70A.040, must adopt the new unit lot subdivision regulations.

While the 2023 legislation applied to “all cities, towns, and counties” throughout the state, now all counties and any cities and towns not within GMA-planning jurisdictions are excluded from the unit lot subdivision requirements. See RCW 58.17.060(3).

New compliance deadlines

Cities and towns that are required to submit their next comprehensive plan periodic update in 2027 pursuant to RCW 36.70A.130 must include unit lot subdivision provisions as part of their next comprehensive plan update. All other cities and towns must implement the requirements within two years of the effective date of the bill, which was May 20, 2025. See RCW 58.17.060(3)(e).

Added definitions

The legislation adopts the following new definitions within RCW 58.17.020, including:

• Parent lot: a residential lot that is subdivided into unit lots through the unit lot subdivision process.
• Unit lot: a subdivided lot within a residential development as created from a parent lot and approved through the unit lot subdivision process.
• Unit lot subdivision: a subdivision or short subdivision proposed as part of a residential development project that meets the development standards applicable to the parent lot at the time the application is vested, but which may result in development on one or more individual unit lots becoming nonconforming as to specified land use and development standards based on the analysis of the individual unit lot. (By June 30, 2026, all unit lot subdivisions shall require notification to purchasers of their legal status as further described in RCW 58.17.060.)
• Clear and objective design and development standards: locally adopted development regulations that involve no personal or subjective judgment by a public official and are ascertainable by reference to measurable written or graphic criteria available and knowable to the permit applicant, the public, and public officials prior to submittal.

Informational notes required on recorded unit lot subdivision plats

Local procedures must include requirements for “prominent informational notes” to be placed on the unit lot subdivision’s plat to be recorded in the county or counties in which the land is located. These required notes are set out in detail at RCW 58.17.060(3)(a)(i)-(iv), and include reference to the original application number, and notes about potential limitations on future development and potential nonconformities of the individual unit lots.

We are assuming these notes reflect the “legal status” of the unit lot subdivisions, which must be provided as a notification to purchasers by June 30, 2026 (see the new “unit lot subdivision” definition at RCW 58.17.020).

Limitations on local review procedures

Local ordinances may not require any public predecision meeting or hearing, nor any design review other than administrative design review, except for those required to comply with state law, such as the Shoreline Management Act at Chapter 90.58 RCW.

Additionally, cities and towns must apply only clear and objective design and development standards to the unit lot subdivision applications. The procedures must also “be logically integrated with the application, review, and approval procedures that apply to the underlying unit lot housing development project to the greatest extent feasible.”

Finally, unit lot subdivisions must be specifically subject to the review timelines at RCW 36.70B.080, unless extended pursuant to project-specific mutual agreement consistent with RCW 36.70B.080. See RCW 58.17.060(b)(i) – (iv).

Notice to community and property owners within 250 feet

Local regulations must include a notice requirement for the community and property owners within 250 feet of the proposed unit lot subdivision consistent with RCW 36.70B.110. This notice must describe how to provide written comments to the administrative decision-maker and must include notice posted on the closest public sidewalk or roadway.

Critical areas and other regulations not impacted

Nothing in the new provisions requires a local government to authorize a unit lot subdivision in a location where development is restricted under other laws or ordinances, such as under a critical areas ordinance.

SB 1096 — Streamlined Residential Lot Splitting in Cities

The legislature also passed a bill intended to facilitate the creation of more buildable lots in cities by making it easier to divide an existing residential lot into two.

In adopting SB 1096, the legislature made the following findings:

The legislature finds that allowing an existing residential lot to be split to create a new residential lot through a simple, administrative process can offer many advantages to both the existing homeowner and to prospective homebuyers. The legislature further finds that administrative lot splitting can provide current owners the opportunity to maintain homeownership in changing life circumstances while facilitating development of middle housing to provide homebuyers, including first-time homebuyers, with more affordable ownership opportunities. The legislature also finds that lot splitting can be combined with the review of a residential building permit application to create a single integrated process benefiting both homeowners and cities. Therefore, it is the intent of the legislature to ease restrictions on, and expand opportunities for, lot splitting in cities.

The bill adds a new section to the state subdivision statute at Chapter 58.17 RCW requiring cities to adopt procedures allowing for an applicant to seek review and approval of an administrative lot split, which may also be combined with concurrent review of a residential building permit to create either middle housing or single-family housing.

The bill also amends the GMA to exempt any lots created through this new residential lot split process from the detailed middle housing requirements at RCW 36.70A.635. Under the new law, any application process for a lot split:

May require only an administrative decision, through which the application is reviewed, approved, or denied by the planning director or other designee based on applicable clear and objective development standards, with neither a pre-decision public hearing, nor any design review other than administrative design review. A new buildable residential lot and residential building permit or permits must be administratively approved and are not subject to administrative appeal if they comply with applicable development standards…

The legislation then sets forth nine conditions that must be met, including that:

• No more than one newly created lot is created through the administrative lot split;
• Both the parent lot and the newly created lot meet the minimum lot size allowed under applicable development regulations;
• The parent lot was not created through the splitting of a residential lot authorized by this section;
• The parent lot be located in a residential zone and not in an exclusively nonresidential zone;
• The applicant must recommend a displacement mitigation strategy that may include, but is not limited to, relocation assistance if the lot split requires demolition or alteration of any existing housing that would displace a renter;
• The applicable sewer and water purveyors have issued certificates of availability to serve the newly created lot and dwelling units; and
• Access and utility rights are granted or conveyed as necessary to provide access for the maximum number of potential dwelling units (unless fewer are contemplated as part of the development).

Note: See the legislation for a full list of the conditions and exact wording.

A city may still require dedication of right-of-way on the parent lot to the extent such a dedication is required under applicable regulations. Likewise, residential development on the lots may be conditioned on construction of frontage improvements to the extent allowed under applicable regulations.

Lots that are otherwise not buildable because of critical areas, shorelines, impervious surface limitations, or other locally adopted development regulations are not eligible for a lot split under these new provisions.

The new law directs the Washington State Department of Commerce to provide guidance for cities required to adopt lot splitting provisions.

Compliance deadline

Cities that are required to submit their next comprehensive plan periodic update in 2027 pursuant to RCW 36.70A.130 must include lot-splitting provisions as part of their next comprehensive plan update. All other cities must implement the requirements within two years of the effective date of the bill, which was May 20, 2025.

Local ordinances adopting new lot splitting provisions are exempt from SEPA administrative or judicial appeal under Chapter 43.21C RCW.

Conclusion

Cities and towns should review these new residential land division provisions with their city and town attorneys and develop a plan to adopt these local regulations within the timeframes set forth in the new laws.