Washington updates the rules on splitting residential land

Washington updates the rules on splitting residential land​

Washington adopted two important land division laws in 2025, which aimed to speed up housing production and make local reviews more predictable. One law updates how cities regulate “unit lot subdivisions.” The other creates a simple process for residential lot splits within cities. Together, they set clearer standards, shortened review steps, and informed cities when they must update their codes.

SB 5559: Clearer rules for unit lot subdivisions
SB 5559 adds detail to the 2023 law, which first required local governments to allow unit lot subdivisions. The new law narrows down the scope of who must comply. Only cities and towns in counties that plan under the Growth Management Act must adopt these rules. Counties and non-GMA jurisdictions are not required to do so.

The bill sets the timelines. Cities that owe a comprehensive plan update in 2027 must include unit lot subdivision provisions in that update. All other affected cities and towns must adopt the required regulations within two years of the bill’s effective date (May 20, 2025).

SB 5559 also adds definitions for “parent lot,” “unit lot,” and “unit lot subdivision,” and defines what “clear and objective” standards mean. It requires prominent informational notes on recorded plats, including the original application number and warnings regarding any limits on future development. Purchasers must be notified of the legal status of a unit lot subdivision by June 30, 2026.

The local review must be administrative. Cities cannot require pre-decision public hearings or design reviews beyond an administrative check, except where state law, such as the Shoreline Management Act, requires it. Review timelines in RCW 36.70B.080 apply, and neighbors within 250 feet must receive notice with instructions on how to submit comments. Critical areas and other protective regulations still apply.
 
HB 1096: A simple, administrative lot split inside cities

HB 1096 directs cities to create a straightforward process for splitting one residential lot into two, if certain conditions are met. The decision is made administratively by the planning director or a designee, using clear and objective standards. The lot split can be reviewed simultaneously with a residential building permit for either single-family or middle housing. Lots created under this process are exempt from the detailed middle housing requirements of RCW 36.70A.635.

The law lists specific conditions. Only one new lot may be created, both lots must meet the minimum size requirement, the parent lot cannot be the product of a prior split under this section, and the site must be in a residential zone. The applicant must propose a strategy to reduce renter displacement if a tenant is affected. Utilities must be available, and access and utility rights must be secured for the number of homes. Lots that are not buildable due to critical areas, shorelines, or similar limits do not qualify for this exemption.

Compliance timelines mirror those of SB 5559. Cities with a 2027 periodic update must include lot split provisions in that update. Other cities have two years from May 20, 2025. Local ordinances adopting these provisions are exempt from SEPA’s administrative or judicial appeals. The Department of Commerce will provide guidance to cities regarding implementation.

Why this matters

For local governments, these laws call for prompt code updates, standardized administrative reviews, and better public notice. For homeowners and builders, the changes open new paths to create additional homes on existing land while keeping reviews focused on clear and measurable rules. The result should be a faster, more predictable process that respects environmental and safety limits.

Washington is pushing for more housing by making land division simpler and clearer. SB 5559 strengthens the unit lot subdivision rules for GMA cities and towns. HB 1096 provides cities with a reliable administrative path to split residential lots when basic standards are met. Both laws set firm timelines and maintain protections for critical areas. To dive deeper into the details and citations, read the original MRSC article: https://mrsc.org/stay-informed/mrsc-insight/june-2025/subdivision-legislation. 
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1.Municipal Research and Services Center. “2025 Legislation Addressing Residential Unit Lot Subdivisions and Lot Splitting.” June 16, 2025. MRSC
2.Washington State Legislature. “SB 5559 — Streamlining the subdivision process inside urban growth areas.” Bill Summary and Status, 2025–26 biennium. Washington State Legislature
3.Washington State Legislature. “HB 1096 — Increasing housing options through lot splitting.” Bill Summary and Status, 2025–26 biennium. Washington State Legislature
4.Washington State Senate Committee Services. “Senate Bill Report: SB 5559.” February 13, 2025. LawFiles
5.Washington State House of Representatives, Office of Program Research. “House Bill Report: E2SHB 1096, As Passed Legislature.” April 2025. LawFiles
6.Washington State Senate Committee Services. “Senate Bill Report: E2SHB 1096, As Passed Senate — Amended.” April 14, 2025. LawFiles
7.Municipal Research and Services Center. “Using Unit Lot Subdivisions to Increase the Local Housing Supply.” February 24, 2025. MRSC
Municipal Research and Services Center. “Subdivisions.” Practice topic page, updated 2025. MRSC

• This article was written with contributions from AI to organize the information and improve its readability.

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