ADU Construction Across King County, WA

The phrase “King County ADU” can describe a project in Seattle, Bellevue, Renton, Burien, an Eastside city or unincorporated land. Those properties do not share one planning desk. King County Local Services handles permits for unincorporated areas; incorporated cities use their own departments and codes.

This page is a jurisdiction hub rather than a claim that one provider serves the entire county. Confirm the parcel authority and current provider coverage before relying on local details.

Is the property inside a city?

Start with the parcel address and jurisdiction mapping, not the postal city alone. A mailing address can use a nearby city name while the property remains unincorporated. King County’s permit service explains that owners inside city limits need to contact that city.

Record:

• parcel number and situs address;
• incorporated city or unincorporated status;
• zoning and overlays;
• water, sewer, power and fire authorities; and
• whether the site is inside the Urban Growth Area.

That information determines which location guide and official material to use.

What does King County say for unincorporated property?

King County Local Services published an updated Residential Accessory Dwelling Units handout in March 2026. It says urban-growth-area properties may have up to two ADUs, attached, detached or combined. Rural areas outside the Urban Growth Area are generally limited to one.

The handout says ADUs are typically limited to 1,000 square feet of heated floor area, with stated exceptions for urban projects, basements and certain unheated area. Rural detached ADUs also depend on zone and minimum lot size. Agriculture and Forest zones have separate limitations.

These are current planning points, not an approval summary. Review the live King County Code and confirm the parcel with the County before choosing a plan.

Why is an old County FAQ not enough?

Some indexed County FAQ text still describes earlier rules, including a one-ADU limit and owner-occupancy language. The March 2026 handout and current code reflect later state and County changes. That mismatch is exactly why source dates matter.

When two official pages disagree, do not choose the more favorable answer. Ask the authority which document controls and save the response with the project file.

What site constraints matter outside Seattle?

Unincorporated King County includes dense urban neighborhoods and large rural properties. The site investigation should address:

• zone and Urban Growth Area status;
• critical areas, buffers and steep slopes;
• shoreline or flood conditions;
• wells, septic systems or sewer availability;
• rural access and fire response;
• driveway and emergency access;
• tree clearing and grading;
• stormwater and erosion control; and
• long utility runs or service upgrades.

On-site sewage can change unit capacity and cost. King County Environmental Health advises owners to discuss buildability with the local building official before designing an on-site sewage system and to distinguish an ADU with a kitchen from accessory living quarters without one.

How do city projects differ?

City of Seattle properties use SDCI and can access Seattle’s ADUniverse plan program. Bellevue, Kirkland and Redmond use their own Eastside codes and permit resources. Renton offers a permit-ready plan program. Shoreline, Burien, Bothell and Mercer Island each publish separate materials.

Use these guides:

• Seattle
• Eastside jurisdictions
• Shoreline
• Renton
• Burien
• Bothell
• Mercer Island

Do not import one city’s size, plan or review promise into another.

What belongs in a King County feasibility brief?

Prepare a short file before requesting prices:

1. jurisdiction, parcel and zoning source;
2. existing dwelling and legal-unit information;
3. proposed attached, detached or conversion path;
4. intended size, rooms and long-term use;
5. site plan or survey information;
6. access, slope, trees and critical-area notes;
7. water, sewer or septic, electrical and drainage facts;
8. plan source and permitted modifications; and
9. decisions or correspondence already received from the authority.

The feasibility guide provides the complete checklist.

How should cost be compared across the County?

A city conversion and a rural detached build can both be called an ADU while carrying very different work. Use the Seattle-area cost guide as a category checklist, not a universal price.

County proposals should separately show design and engineering, permits and fees, clearing and grading, foundation, building work, utilities or septic, driveway and fire access, stormwater, finishes, restoration and contingency. Ask the same providers to price the same property information and design basis.

How should a regional builder be checked?

Washington L&I lets owners search the legal contracting entity and view registration, bond, insurance and other records. Recheck the registration before signing. Then request recent projects under the same city or County authority, not only a broad Puget Sound portfolio.

Ask who handles planning responses, structural design, energy documents, utility coordination and site work. Confirm that the advertised company is the entity named in the proposal. Use the builder comparison to normalize evidence and contract terms.

Which County decisions should be closed before design?

For unincorporated property, obtain written answers where the parcel raises an urban-growth boundary, rural lot-size, critical-area, shoreline, septic or access question. Record whether the proposed unit is attached, detached or a conversion and whether another accessory unit already exists. A general “ADUs are allowed” answer does not settle those project facts.

Also identify the reviewing departments and outside agencies. The building permit, on-site sewage review, water service, fire access and road or right-of-way work may not be handled by one desk. Put each contact, application and unresolved condition into the schedule so a building-plan milestone is not mistaken for complete project clearance.