Unit Lot Subdivisions in Washington State

A New Path for Small-Scale Development

In recent years, Washington State has taken major steps to encourage housing diversity and streamline the subdivision process for small-scale projects. One of the most significant tools to emerge is the Unit Lot Subdivision (ULS)—a flexible legal mechanism that enables the creation of individually owned lots within a shared parent parcel, often used for townhomes or cottage clusters.

With statewide changes now in effect and local governments implementing new procedures, it’s important to understand how Unit Lot Subdivisions work, what’s required, and how they differ from traditional subdivisions.

What Is a Unit Lot Subdivision?

A Unit Lot Subdivision allows a single parcel (the “parent lot”) to be divided into smaller lots (“unit lots”) for the purpose of individual ownership—without requiring each unit lot to meet all traditional zoning standards (like minimum lot size or frontage). The key is that the overall parent lot must meet all applicable code requirements, while the unit lots may be smaller and more flexible.

This is particularly useful in urban infill areas or for developments like:

  • Townhomes

  • Cottage housing

  • Duplexes, triplexes, and fourplexes

  • Small-scale multifamily projects

Why It Matters

The Washington State Legislature formalized the process through ESSB 5258, signed into law in 2023. As of May 2025, all jurisdictions are required to allow and process Unit Lot Subdivisions under their short plat procedures, streamlining the process and removing many barriers that previously existed for small-scale developers and builders.

Key Benefits

  • Faster approvals (processed as short plats)

  • Increased housing options in existing neighborhoods

  • Ownership opportunities for attached and clustered homes

  • More efficient use of land, especially in urban cores

Local Jurisdictions with ULS Guidance

Several Washington cities and counties have already implemented ULS-specific standards. Here are helpful links to local information:

What’s Required?

While each jurisdiction may have slight variations, most Unit Lot Subdivisions in Washington require:

  • Short Subdivision application

  • Survey and legal descriptions of the unit lots

  • Common ownership agreements for shared infrastructure or open space

  • Plat notations explaining the parent lot and unit lot relationship

  • Disclosure to future buyers about the nature of the subdivision

Importantly, these subdivisions do not exempt a project from design, stormwater, critical area, or other applicable regulations.

How McInnis Can Help

Navigating the evolving rules around Unit Lot Subdivisions can be complex, especially as jurisdictions interpret the new state laws. Our team at McInnis Engineering is actively working with cities and counties across the Puget Sound region to process these new subdivisions efficiently and accurately.

Whether you're a builder, developer, or property owner looking to divide land for attached housing, we can help with:

  • Site feasibility evaluations

  • Short plat applications

  • Engineering and grading plans

  • Coordination with city staff and reviewers

We stay up to date on local code changes so you don’t have to.

Final Thoughts

As housing affordability and density continue to drive planning conversations in Washington, Unit Lot Subdivisions are becoming an increasingly important tool for property owners and developers. If you’re considering subdividing a parcel into multiple ownership opportunities—without the cost and complexity of traditional subdivision—ULS might be the right path.

For more information about your specific site or jurisdiction, feel free to reach out. McInnis can help you understand what’s possible under today’s evolving land use rules.