As the housing market across the Northwest continues to tighten, many homeowners are looking for creative ways to add value to their property and flexibility to their lifestyles. Enter the Accessory Dwelling Unit (ADU). Whether you call it a “granny flat,” a “backyard cottage,” or a “shop-house,” these secondary units are becoming the ultimate solution for modern homeowners.
With landmark legislative changes recently passing in Idaho, Washington, and Montana, it has never been easier or more legal to build. Here is everything you need to know about the benefits of ADUs and the specific rules in your region.
What is an ADU?
An ADU is a secondary, self-contained living space located on the same lot as a primary single-family home. They generally fall into two categories:
- Internal/Attached: A basement apartment, a converted garage, or an addition to the side of the house.
- Detached: A completely separate, freestanding structure in the backyard.
The Pros & Cons of Adding an ADU
The Pros (The Benefits)
- Passive Rental Income: ADUs provide a steady stream of monthly income through long-term rentals.
- Multi-Generational Living: They offer a private space for aging parents or adult children to live nearby while maintaining independence.
- Increased Property Value: Most properties with a legal ADU see a significant boost in resale value.
- Flexible Space: When not being rented, an ADU serves as a high-end guest suite, a quiet home office, or a creative studio.
The Cons (The Challenges)
- Upfront Investment: Building a quality ADU involves significant costs, including permits, site prep, and construction.
- Increased Taxes: A higher property value usually results in a corresponding increase in property taxes.
- Landlord Responsibilities: If you rent the unit, you are responsible for maintenance, repairs, and managing tenants.
- Yard Sacrifice: A detached ADU will occupy a portion of your backyard or garden space.
Regional Breakdown: The New Rules for 2026
Recent state laws have stripped away much of the “red tape” that used to block ADU projects. Here is how the rules stack up in our primary service areas:
1. Idaho: A New Era for Homeowners
The landscape in Idaho changed dramatically with Senate Bill 1354, which takes full effect July 1, 2026.
- Applies To: Cities with populations over 10,000 (Boise, Coeur d’Alene, Post Falls, Meridian, etc.).
- One ADU Per Lot: Cities must allow at least one ADU (internal or detached) as a “matter of right.”
- Size Protections: Cities cannot limit an ADU to less than 1,000 square feet or 75% of the primary home’s size (whichever is larger).
- No Owner-Occupancy: You are no longer required to live on-site to rent out your ADU.
- HOA Note: Under HB 166, HOAs cannot ban internal ADUs on owner-occupied properties, though pre-existing CC&R restrictions may still apply.
2. Eastern Washington: Double the Opportunity
Washington remains one of the most “ADU-friendly” states in the country thanks to HB 1337.
- Two ADUs Allowed: In most urban areas (like Spokane and Kennewick), you can now build up to two ADUs on a single lot—for example, one attached and one detached.
- Bigger Units: Washington mandates that cities allow ADUs up to at least 1,000 square feet.
- Lower Fees: The state has capped impact fees for ADUs at 50% of the standard rate for a single-family home.
- Parking: Cities generally cannot require additional off-street parking for ADUs located within a half-mile of transit.
3. Western Montana: Protecting Your Rights
In response to rapid growth in Missoula, Bozeman, and Kalispell, Montana passed SB 528, creating a baseline for property rights.
- The “Right to Build”: Municipalities must allow at least one ADU on any lot where a single-family home exists.
- Aesthetic Freedom: The law prevents cities from imposing “unreasonable” design or material requirements that drive up costs.
- No Parking Mandates: Like its neighbors, Montana limits the ability of cities to require extra parking spaces, particularly in higher-density areas.
Conclusion
Whether you are looking to generate retirement income or keep your family close, the new laws in Idaho, Washington, and Montana have cleared the path. However, local “setbacks” (the distance from the property line) and utility connections still vary by city.
Ready to start? Contact our regional agents today to discuss the specific zoning requirements for your backyard!