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Washington law now requires many cities and counties to allow more accessory dwelling unit options in urban growth areas. That does not mean every lot can support the same ADU. Local zoning, lot conditions, utilities, critical areas, shoreline rules, building codes, and the proposed design still determine what can actually be permitted.
Washington law now requires many cities and counties to allow more accessory dwelling unit options in urban growth areas. That does not mean every lot can support the same ADU. Local zoning, lot conditions, utilities, critical areas, shoreline rules, building codes, and the proposed design still determine what can actually be permitted.
This guide explains the statewide framework and the local checks that should happen before a homeowner commits to an ADU design or construction budget.
What is an ADU in Washington?
An accessory dwelling unit, or ADU, is a smaller independent dwelling located on the same lot as a principal home. It normally includes its own sleeping area, kitchen, bathroom, and living space.
Common forms include:
- an attached addition connected to the main house;
- a conversion within a basement or another part of the house;
- a conversion of an existing garage or accessory structure; or
- a new detached accessory dwelling unit, often called a DADU.
The form matters because a basement conversion, garage conversion, addition, and detached building can trigger different structural, energy, fire separation, utility, access, and site-planning questions.
What does Washington State law require?
RCW 36.70A.681 establishes statewide limits on how local governments may regulate ADUs in urban growth areas. Among other provisions, it requires covered jurisdictions to allow at least two ADUs per lot in qualifying residential zones and restricts certain owner-occupancy and parking requirements. The statute also contains conditions, deadlines, and exceptions, so it should not be reduced to a single universal rule for every property.
RCW 36.70A.680 contains related definitions and planning requirements. Homeowners should read the state framework together with the current code and permit guidance for the authority that governs their address.
State rule versus local review
| Planning question | Statewide framework | What remains local or site-specific |
|---|---|---|
| Number of ADUs | State law requires broader allowance in covered urban growth areas | Whether the lot and proposed arrangement satisfy local standards |
| Owner occupancy | Broad owner-occupancy mandates are restricted | Statutory exceptions and lawful local conditions |
| Parking | Some off-street parking mandates are restricted | Location-specific exceptions and existing site constraints |
| ADU size | State law sets limits on restrictive local rules | The applicable city or county size standard and design details |
| Permits | ADUs must be allowed under the state framework | Building, land use, utility, septic, electrical, and other required approvals |
| Construction | State law affects land-use allowance | Building code, structural design, energy code, fire safety, and inspections |
Can a Washington property have two ADUs?
Many lots in urban growth areas must be allowed at least two ADUs under the state framework. The configuration may include attached and detached units, but the exact options depend on the applicable local code and the property.
Seattle states that a lot with one principal dwelling may have up to two ADUs, subject to current standards. The city also publishes a dedicated ADU permit guide and standard plans.
For unincorporated King County, the county's March 2026 residential ADU information sheet distinguishes between urban and rural areas and directs property owners to verify rules using the parcel and address. An older handout or blog post should not be treated as the current rule when newer county guidance is available.
What must homeowners verify before designing an ADU?
Which authority has jurisdiction?
The mailing address is not always enough to identify the permitting authority. A property may be inside an incorporated city or in unincorporated county territory. Confirm the parcel's jurisdiction before relying on city or county rules.
Is the property inside an urban growth area?
The statewide ADU requirements apply differently depending on location. Verify whether the parcel is inside an urban growth area and whether any statutory exception affects it.
What can fit on the lot?
Check setbacks, height, lot coverage, access, tree protection, easements, drainage, steep slopes, wetlands, shoreline areas, and other mapped constraints. A zoning allowance does not guarantee that a detached unit of a particular size will fit.
How will utilities work?
Confirm water, sewer or septic capacity, stormwater requirements, electrical service, and any utility connection or upgrade costs. For onsite sewage systems, coordinate early with the responsible health authority. King County Public Health publishes separate onsite sewage guidance for ADUs.
What permits and reviews are required?
An ADU project may require building, land-use, mechanical, plumbing, electrical, right-of-way, utility, or environmental review. The list depends on the address and scope. Do not order a permit set from a generic checklist before the jurisdiction confirms the required submittals.
How large can an ADU be?
There is no single size that applies to every Washington property. Local rules must comply with the state framework, but the adopted code still controls the permit application.
For example, Seattle's current Neighborhood Residential Code guidance links to the city's adopted zoning provisions, including ADU standards. Unincorporated King County publishes its own standards in the county handout. Other cities in King and Snohomish counties may use different tables, definitions, or design requirements.
The practical rule is simple: verify the current code for the parcel before fixing the floor area, height, or footprint.
Does an ADU require owner occupancy or extra parking?
Washington law restricts broad owner-occupancy requirements and limits when jurisdictions can require additional off-street parking. Those protections are important, but a homeowner should still review the exact statutory language and local implementation. Transit proximity, geographic exceptions, existing parking arrangements, recorded documents, and other site facts may matter.
Avoid treating a summary from another city as permission for your own lot.
What should happen before construction pricing?
An early ADU feasibility phase should establish:
- jurisdiction and zoning;
- allowed ADU configuration;
- conceptual placement or conversion scope;
- utility and septic assumptions;
- critical-area and shoreline screening;
- structural and access constraints;
- required consultants and permit path; and
- a preliminary scope with clearly stated allowances and unknowns.
This sequence reduces the risk of pricing a design that cannot be permitted or that requires unplanned utility and site work.
Frequently asked questions
Do Washington ADU laws apply to every residential lot?
No. The state framework contains location requirements, conditions, and exceptions. Local zoning and site constraints still apply. Verify the property with the responsible city or county.
Do I still need a building permit for an ADU?
Yes. Statewide land-use reform does not remove building-code review, trade permits, inspections, or other approvals required for the project.
Can I convert a garage into an ADU?
Possibly. The structure and site must be reviewed for zoning, foundation and framing, energy code, fire separation, utilities, ventilation, egress, and other applicable requirements.
Can I use Seattle's ADU rules for a Bellevue, Everett, or Kirkland property?
No. Use Seattle guidance only for properties under Seattle jurisdiction. Other cities and unincorporated areas have their own permit authorities and adopted codes.
When should I contact an ADU contractor?
Contact a design-build team early enough to coordinate feasibility, design, consultants, pricing, and permits. The first conversation should identify unknowns, not promise approval before the parcel has been reviewed.
Plan an address-specific ADU review
Alta Group Remodeling plans and builds attached ADUs, detached ADUs, garage conversions, and other residential projects in Seattle, Everett, and surrounding Washington communities. Start with an address-specific review so the design responds to the actual jurisdiction, lot, utilities, and construction scope.
Explore ADU construction services or call 206-773-7361 to discuss the property.
Sources
- Washington State Legislature: RCW 36.70A.680
- Washington State Legislature: RCW 36.70A.681
- Seattle Department of Construction and Inspections: Accessory Dwelling Units
- Seattle Department of Construction and Inspections: Neighborhood Residential Code
- King County: Residential Accessory Dwelling Units, March 2026
This article provides general planning information, not legal or permit advice. Codes and procedures change. Confirm current requirements with the authority having jurisdiction before design or construction.
