Washington ADU (accessory dwelling unit) Laws in 2026: A Practical Guide for Homeowners

Washington did change the rules on backyard cottages. Over the last few years the state has pushed cities and counties to treat accessory dwelling units as a normal use of a residential lot rather than a rare exception. Most homeowners have more room to build than they realize, but the details still depend on where your lot sits.

Here’s what changed and what it means if you’re planning an ADU in Washington.

The statewide change: HB 1337

In 2023 the Legislature passed House Bill 1337, now codified mainly in RCW 36.70A.680 and 36.70A.681. It applies to cities and counties that plan under the Growth Management Act, which includes Clark County and Vancouver.

The law sets minimum allowances for covered development inside urban growth areas. Designated critical-area lots, buffers, and certain protected watersheds are excluded from RCW 36.70A.681. RCW 36.70A.680 also preserves health and safety restrictions and allows local sewer-connection requirements. Confirm the parcel before relying on these general allowances:

• Two ADUs on qualifying lots, with attached, detached, or mixed arrangements.
• Local size caps of at least 1,000 gross square feet. Buildable area still depends on the site.
• No owner-occupancy mandate for covered urban ADUs.
• Height limits of at least 24 feet, or the principal unit’s permitted height if that is lower.
• Comparable development standards: ADU setbacks, coverage, tree retention, entry locations, and design review cannot be more restrictive than the principal unit’s.
• Parking protections: generally no required off-street parking within a half-mile walking distance of a major transit stop. Elsewhere, lot-size caps apply. Check subsection (2), including its certified-safety-study and airport exceptions, with the local reviewer.
• Impact fees no higher than half the principal unit’s impact fees.

The current RCW 36.70A.680 ties implementation to each jurisdiction’s next periodic comprehensive plan update; there is no single statewide mid-2025 deadline. Where required local changes have not been made, the statute preempts conflicting development regulations. Verify the jurisdiction’s applicable update and current permit requirements.

HB 1337’s strongest protections apply inside urban growth areas. Rural lots outside a UGA follow county rural ADU rules and may also be limited by septic capacity.

Clark County and Vancouver

Statewide minimums set the baseline, but the permit comes from the local jurisdiction. Unincorporated Clark County and the City of Vancouver use separate codes. For the 2026 city-versus-unincorporated and UGA-versus-RADU table, see the Clark County ADU rules guide.

City of Vancouver, WA

Vancouver’s updated zoning took effect July 31, 2026. Its current ADU guidance provides:

• Up to two ADUs can sit on a lot alongside the primary dwelling.
• 1,000 sq ft maximum per ADU. The minimum just has to meet the Washington State Building Code.
• No owner-occupancy requirement. You don’t have to live on the property.
• Development standards based on the principal dwelling’s building type.
• No minimum vehicle parking requirement, with standards for any parking the owner chooses to provide.

Use the city’s current New Housing or Dwelling Unit, including ADU application guidance, including when a remodel creates an ADU. Complete the Request for Utility Services review first unless a stated exemption applies. The city also publishes a dedicated modular-home checklist. Confirm the required documents before submitting.

Unincorporated Clark County

Outside city limits, the key question is whether the ADU is urban or rural.

Urban ADUs apply on UGA lots that already allow single-family, duplex, triplex, or townhome dwellings. These follow the HB 1337-aligned rules, including the two-per-lot allowance.

Rural ADUs (RADUs) are governed by Clark County Code 40.260.022 and live in the rural and resource zones (RC-1, RC-2.5, R-5, R-10, R-20, AG-20, FR-40, FR-80, AG-WL). The rural numbers are different from the urban set:

• The published county path is one attached RADU, within or added to the primary home; it does not establish a detached backyard-cottage allowance.
• The usual maximum is 1,500 sq ft of interior area, counting nonhabitable space and excluding garages. A separate basement-conversion exception applies.
• The combined primary-plus-RADU footprint still has to meet the zone’s lot coverage and setbacks.
• HB 1337’s two-ADU mandate does not automatically extend to these rural lots. Rural ADU allowances are a county matter.

Setbacks and height

Start with the zone and, in Vancouver, the principal dwelling’s building type. New construction, conversions, and alley sites can have different requirements. The state height protection above applies to covered urban ADUs; it is not a blanket rural allowance.

Utilities and septic on rural lots

On a sewered city lot, utilities are usually straightforward. On a rural Clark County lot, your septic system often decides whether an ADU is even feasible.

The county requires Public Health’s Septic System Release / Water Adequacy Verification (WAVE) before issuing a rural ADU building permit. Existing approvals, the proposed dwelling, and site conditions determine the review. A system serving the house may need expansion or replacement; an online per-person calculation cannot confirm capacity. Scope the water and wastewater work before choosing a floor plan.

How a factory-built (modular) ADU is treated

A modular ADU is not a HUD (the federal standard for manufactured homes) manufactured home. HUD homes follow a federal construction code. Washington modular homes follow the applicable state building requirements and the L&I approval process.

Washington L&I reviews factory plans and oversees factory inspections. The building must have its L&I insignia before shipping. That approval does not establish that a particular lot can accept the home.

Local review covers placement, the approved foundation design, utility connections, and applicable site inspections. Vancouver’s modular checklist calls for a site plan, L&I-stamped construction plans, and a foundation plan for a private lot. Coordinating factory and site requirements early helps avoid redesign.

How Wolf handles Washington projects

We build in a factory in Battle Ground and work through the L&I process on every home. Our units carry the state insignia when they leave the floor, and we handle the local site permitting in-house. On most projects that means we pull the zone standards, check setbacks and height, and coordinate the Public Health septic review before anything gets built.

Not every lot can support an ADU. Rural septic capacity, zoning, access, or an easement may prevent the project. We review feasibility early, and the permitting agreement explains the refund that applies if the project cannot be permitted.

Send us your address and we’ll tell you whether the lot is urban or rural, city or county, and what size and placement are likely to work.

This is general information from Wolf Industries, not legal advice. Recheck the current statute, local code, and permit packet before you file. Jurisdictions amend forms and effective dates.