The short answer: Washington State ADU law requires cities and counties inside urban growth areas to allow two ADUs per residential lot (RCW 36.70A.680 and 36.70A.681). Local governments cannot require owner occupancy, cannot require parking within a half mile of major transit, and cannot impose extra setbacks, design review or lot-coverage limits that apply only to ADUs. Since June 11, 2026, a separate law (EHB 1345) also opens a limited path for detached ADUs outside urban growth areas.
Last verified: October 2, 2026 ยท Written by the ADU Scope editorial team
Key fact: The statewide rules are a floor. Your city may be more generous, but it may not be stricter than state law for the topics below. Local codes that predate the mandate can still be on the books, so always check the current city page.
Washington State ADU Law: The Statewide Rules at a Glance
Washington’s ADU mandate covers cities and counties planning under the Growth Management Act (GMA). Inside an urban growth area (UGA), the state sets minimum allowances that local ordinances must respect. The table below summarizes what the state requires, based on the Municipal Research and Services Center (MRSC) summary of the statute.
| Topic | What the state requires inside a UGA |
|---|---|
| Number of ADUs | Two per lot on lots that allow a single-family home, in any combination of attached, detached or conversion |
| Size | A city cannot set a maximum smaller than 1,000 sq ft |
| Parking | No parking requirement within one half mile of a major transit stop |
| Owner occupancy | Cannot be required |
| Setbacks, yard coverage, tree retention | Cannot be stricter than those for the main house |
| Design review | No ADU-specific design review beyond what applies to the main house |
| Impact fees | Capped at 50% of the fee for the principal unit |
| Separate sale (condo conveyance) | Cities may not prohibit it for a unit originally built as an ADU |
These items matter because they remove the most common local obstacles. Before the mandate, many Washington cities required the owner to live on site, capped ADUs at a few hundred square feet, or demanded a parking space per unit. Those rules are now largely unenforceable inside UGAs.
How Many ADUs Does Washington State ADU Law Allow?
The state sets a floor of two ADUs per lot. That means you can combine, for example, a basement conversion with a detached backyard cottage, or build two detached units, as long as the lot allows a single-family home to begin with.
Allowed combinations
- Attached ADU: built inside or onto the main house, such as a basement apartment or an addition.
- Detached ADU (DADU): a separate backyard cottage.
- Conversion: a garage or other existing structure turned into a dwelling.
The statute covers combinations in any mix, so a city cannot force you to choose attached over detached when both fit within the ordinary building envelope. What it cannot guarantee is that every lot physically fits two units. Utility capacity, critical areas such as wetlands and steep slopes, and fire access can still limit what is buildable on a specific parcel.
Tip: Ask your planning department for a written pre-application review that lists which unit types and sizes it will accept on your parcel. It costs little and prevents expensive redesigns.
Washington State ADU Law: Size, Setbacks and Design Standards
Inside a UGA, a city cannot require ADUs to be smaller than 1,000 sq ft. Many cities set the cap exactly at 1,000 sq ft, but some allow more, so a larger home-sized ADU can be legal locally even if the state floor is lower.
The more important protection is that local governments cannot apply stricter standards to ADUs than to the main house for setbacks, yard coverage, tree retention, or entry-door location. They also cannot impose ADU-specific design review. In practice this means an ADU follows the same objective development standards as the house in front of it.
What local governments can still control
- Building, fire and energy codes that apply to any dwelling.
- Critical areas rules (wetlands, floodplains, steep slopes, shorelines).
- Permit and plan-review fees, which vary by city and county.
- Utility and sewer capacity requirements.
Because these remain local, two lots with identical zoning can have different timelines. A lot in a shoreline area or near a stream, for instance, may need a critical-areas review that a flat inland lot avoids entirely.
Washington State ADU Law on Parking, Owner Occupancy and Rentals
Parking is one of the biggest cost drivers for an ADU, and Washington limits it. Inside a UGA, a city cannot require parking for an ADU within one half mile of a major transit stop. Outside that radius, local parking rules may still apply, but newer state parking reform (RCW 35.21.994) bars minimum parking requirements for homes under 1,200 sq ft in cities over 30,000 residents, phased in by city size (cities of 50,000 or more by January 27, 2027; cities of 30,000 to 50,000 by July 27, 2028). Check the half-mile test and your city’s parking rules.
Owner occupancy is also off the table. Cities cannot require you to live in either the main house or the ADU. That makes it possible for an investor to own the lot and rent both units, or for a family to rent out the main house and live in the backyard cottage.
Attention: “No owner-occupancy requirement” does not mean “no rental rules.” Washington’s landlord-tenant law, local rental registration and short-term rental ordinances still apply. Confirm local short-term rental limits before you plan an ADU around nightly rentals.
Impact Fees and Selling an ADU Separately
Impact fees. Local governments may not charge ADU impact fees above 50% of the fee for the principal unit. Fees are paid at permit time and can run into the thousands, so the cap is meaningful. It applies only to impact fees (parks, schools, transportation, fire). It does not cap permit fees, utility connection charges, or sewer capacity charges, which are set separately.
Selling a unit separately. The state bars local governments from prohibiting condominium-style conveyance of a unit that was originally built as an ADU. That opens a path to sell the cottage separately, but it is not automatic. You still need a condominium declaration, a survey, lender cooperation, and compliance with building and fire separation rules. Treat it as a project of its own.
| Cost item | Capped by state law? |
|---|---|
| Impact fees | Yes, 50% of the principal-unit fee |
| Permit and plan-review fees | No, set locally |
| Utility connection and sewer capacity charges | No, set by the utility |
New in 2026: Detached ADUs Outside Urban Growth Areas
EHB 1345 (Chapter 231, Laws of 2026) took effect on June 11, 2026. It lets counties allow detached ADUs on rural land outside UGAs, under tighter conditions than the urban mandate. The final bill report describes these main standards:
- One ADU per parcel, attached or detached combined.
- Maximum 1,296 sq ft of gross floor area, excluding garages and porches.
- Within 150 feet of the main house and sharing the same driveway.
- Metered water use, with combined household use within state domestic-use limits.
- Septic documentation showing the system can handle the added demand.
- Not allowed on nonconforming lots under one acre.
The law also builds in enforcement. Counties that allow these ADUs must offer a voluntary compliance process with double permit fees. Noncompliant owners face a civil infraction of at least $1,000, and triple permit fees if the unit stays out of compliance. Owners who refuse voluntary compliance can be barred from new ADU permits for at least three years.
Tip: If you own rural land, check your county’s comprehensive plan update before you design. This is a county option: counties may allow these ADUs, and may allocate no more than 10% (rural counties) or 7% (other counties) of their rural population target to them, so availability differs county by county.
Compliance Deadlines and Why Your City May Still Look Different
Cities and counties must adopt these rules when their next periodic comprehensive plan update takes effect (RCW 36.70A.680). Those deadlines are December 31, 2024 for King, Kitsap, Pierce and Snohomish counties; December 31, 2025 for Clallam, Clark, Island, Jefferson, Lewis, Mason, San Juan, Skagit, Thurston and Whatcom; December 31, 2026 for Benton, Chelan, Cowlitz, Douglas, Franklin, Kittitas, Skamania, Spokane, Walla Walla and Yakima; and June 30, 2027 for the remaining counties. Until a city updates, the state rules preempt conflicting local limits, but the city website may show outdated ones.
What to do if your city’s page contradicts state law
- Print the relevant RCW sections (36.70A.680 and 36.70A.681).
- Ask the planning counter which provision of the code the requirement comes from.
- Request a written determination if staff insist on the older rule.
- Consult a local land use attorney before paying for plans based on a disputed requirement.
Frequently Asked Questions
Can I build two ADUs on one lot in Washington?
Inside an urban growth area, yes. State law requires cities to allow two ADUs on lots where a single-family home is allowed. Physical constraints can still limit what fits.
Do I need to live on the property?
No. Washington cities cannot require owner occupancy for ADUs.
Do I need to provide parking?
Not within a half mile of a major transit stop. Elsewhere, local rules may apply.
Can I build an ADU on rural land?
Possibly, under EHB 1345 and your county’s rules. The law allows one ADU per parcel with size, distance, water and septic limits.
Are there fee caps?
Impact fees are capped at 50% of the principal unit’s fee. Permit and utility charges are not capped.
Next Steps
- ADU Laws by State: Where You Can Build in 2026
- Can I Build an ADU on My Property? A 5-Step Checker
- ADU Impact Fees: When You Pay Them and How to Avoid Them
- Oregon ADU Rules: Statewide Requirements and Portland Specifics
- ADU Setback Requirements Explained: California, Washington, Colorado and Portland
- Can My HOA Block an ADU? Rules by State
General information, not legal advice. Local rules vary and change; confirm with your planning department.
Sources: MRSC: Accessory Dwelling Units ยท Final Bill Report, EHB 1345 (C 231 L 26) ยท RCW 36.70A.680 ยท RCW 36.70A.681