ADU Rules in Kittitas County: A 2026 Guide for Homeowners

If you own land in Kittitas County and you've thought about adding an Accessory Dwelling Unit (a small second home for rental income, aging parents, an adult child, or a future guest house) 2026 is a moment worth paying attention to. Washington State has rewritten the rules around ADUs, Kittitas County is in the middle of a Comprehensive Plan update due by the end of 2026, and the gap between what state law requires and what local code currently says has never been wider.
This guide covers what's allowed today, what's changing soon, and the practical questions you'll need to answer before you start designing. None of it is legal advice; every property is different, and the only authoritative source for your specific parcel is Kittitas County Community Development Services. But knowing the right questions to ask is half the battle.

First, the most important distinction: UGA or rural?
Kittitas County's ADU rules split into two very different worlds depending on whether your property sits inside an Urban Growth Area (UGA) or in unincorporated rural land.
Urban Growth Areas in Kittitas County include the city limits and adjacent expansion zones around Cle Elum, South Cle Elum, Roslyn, Ronald, Easton, Ellensburg, and Kittitas. If your address has city water and city sewer, you're almost certainly inside a UGA. If you're on a well and septic with several acres, you're almost certainly outside one.
This matters because Washington's House Bill 1337, the major 2023 ADU reform, only applies inside UGAs. Outside, the older county rules still govern.

What HB 1337 changed (inside UGAs)
HB 1337 took effect statewide in 2023, with cities and counties required to bring their codes into compliance through 2024 and 2025. For lots inside an Urban Growth Area, the law requires local governments to:

• Allow at least two ADUs per lot (in any combination of attached, detached, or conversions of existing structures).
• Allow ADUs of at least 1,000 square feet; local code can permit larger but cannot cap below this.
• Eliminate owner-occupancy requirements. You no longer have to live on the property to build or rent an ADU.
• Cap impact fees on ADUs at no more than 50% of what's charged for the primary unit.
• Limit off-street parking to one space per ADU, with no parking required at all within a half-mile of a major transit stop.
• Allow building heights of at least 24 feet for ADUs (or up to whatever the primary residence is allowed).

In practice, this means a UGA homeowner in Cle Elum or Ellensburg now has significantly more flexibility than they did a few years ago. The City of Cle Elum and the City of Ellensburg are working through their code updates to bring local zoning into HB 1337 compliance. If you're inside city limits, confirm the current city code, not just the county code, since the city is your permitting authority.

What the rules look like outside a UGA
Most of Kittitas County by land area sits outside any UGA, including most of Suncadia, Tumble Creek, Ronald, the Teanaway, and the rural areas surrounding Cle Elum and Ellensburg. Out here, HB 1337 does not apply (at least not yet), and Kittitas County Code Title 17 still governs.

Under the existing county code for rural and resource lands, the typical rules look like this:

• ​One ADU per lot (3 acre minimum) is generally allowed, often as a conditional use rather than an outright permitted use.
• The owner of the property must live in either the primary residence or the ADU.
• The ADU must meet all setback requirements for its zone, and in some zones (Commercial Forest, for example) those setbacks can be 200 feet from every property line.
• The ADU must meet Kittitas County Health Department standards for water and septic. This is often the controlling constraint on rural lots.
• Mobile homes and recreational vehicles are not allowed as ADUs.
• Additional off-street parking is required.
• An ADU is not allowed on a lot that already has a Special Care Dwelling or Accessory Living Quarters.

Rural ADU permits typically also require a land use permit in addition to the building permit, which adds time and cost to the front end of a project.

The Comprehensive Plan Update: What to Watch in 2026
Kittitas County is required to complete its periodic Comprehensive Plan update by December 31, 2026. As part of that update, the county must address ADU regulations alongside middle housing and design review. This is the window in which rural ADU rules could meaningfully change, though exactly how is still being worked out at the planning commission level.
If you're planning a rural ADU project for 2026 or 2027, it's worth keeping an eye on the county's draft regulations as they're released. A project designed to current rules may be designable to better rules within months.

The septic question
For most rural Kittitas County properties, the binding constraint on whether an ADU is even possible isn't zoning. It's the septic system. Adding a second dwelling unit means adding bedrooms, which means proving the existing septic system can handle the additional load, or designing a system upgrade or a separate septic system for the ADU.
Before any ADU design work begins on a rural property, the recommended first step is a conversation with the Kittitas County Environmental Health Department about your existing system's capacity and any required upgrades. A septic feasibility check costs little and can save you from designing an ADU you can't actually permit.

What a typical Kittitas County ADU project looks like
From first conversation to permit submittal, a Kittitas County ADU typically takes 6--10 weeks of design time, depending on site complexity, septic engineering, and how quickly the structural engineer can turn around calculations. Construction time depends on the builder, but a 600--900 square foot detached ADU usually frames and finishes in 4--6 months once permitted.

A complete permit submittal for an ADU in Kittitas County generally includes:

• Site plan showing setbacks, easements, septic, well, and any critical areas.
• Floor plans for both the primary residence and the proposed ADU.
• Building elevations and section drawings.
• Foundation plan and structural engineering.
• Energy code compliance worksheets per the Washington State Energy Code.
• Septic and well documentation from Environmental Health.
• Plans drawn at quarter-inch scale on minimum 11x17 paper, with two complete sets submitted.

How Henrichsen Design helps
As a Cle Elum based architectural design firm, Henrichsen Design has been producing ADU drawings for Kittitas County properties for years. That includes detached ADUs on rural acreage, garage conversions in Cle Elum and Roslyn, basement ADUs on sloped lots, and attached ADUs on smaller in-town parcels. Each project starts with a conversation about what you're trying to accomplish, what your site allows, and which of the dozens of small decisions in an ADU design will most affect cost, livability, and permittability.
If you're considering an ADU on your Kittitas County property, the best first step is a no-cost initial consultation. Call 509-260-0614 or fill out the Design Questionnaire on the Henrichsen Design website to share basic project details: lot location, what you're hoping to build, and any constraints you already know about. You'll hear back within one business day.