ADU Construction Across King County, WA

“King County ADU” may mean a project in Seattle, Bellevue, Renton, Burien or an Eastside city. It can also mean land outside city limits. These places do not share one permit desk. King County Local Services handles unincorporated areas, while cities use their own codes and offices.

This page is a city or county hub rather than a claim that one company serves the entire county. Confirm the parcel permit office and current company coverage before relying on local details.

Is the site inside a city?

Start with the parcel and official boundary map, not the mailing city. A mailing address may use a nearby city name while the land sits outside city limits. If the property is inside a city, contact that city.

Record:

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

Those details decide which location guide and official material to use.

What does King County say for unincorporated property?

King County Local Services published an updated ADU handout in March 2026. It says land in the Urban Growth Area may have up to two ADUs. They may be attached, detached or both. Rural areas outside that boundary are generally limited to one.

The handout generally limits ADUs to 1,000 square feet of heated space. It lists exceptions for some urban projects, basements and unheated areas. Rural detached ADUs also depend on zoning and lot size. Farm and Forest zones have different limits.

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 older County FAQ text still shows past rules, such as a one-ADU limit and owner-occupancy terms. The March 2026 handout and current code reflect later changes. This is why the source date matters.

When two official pages disagree, do not choose the more favorable answer. Ask the permit office 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 review 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.
• long utility runs or service upgrades.

Septic can change how many homes the site can support and what they cost. King County Environmental Health tells owners to discuss buildability with the local building official before designing the system. It also separates an ADU with a kitchen from 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 different materials.

Use these guides:

• Seattle
• Eastside cities and counties
• 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 site check brief?

Prepare a short file before asking for prices:

1. City or county, parcel and zoning source.
2. Current dwelling and legal-unit details.
3. Planned attached, detached or conversion path.
4. Intended size, rooms and long-term use.
5. Site plan or survey details.
6. Access, slope, trees and critical-area notes.
7. Water, sewer or septic, electrical and drainage facts.
8. Plan source and permitted changes.
9. choices or correspondence already received from the permit office.

The early site-check guide provides the complete checklist.

How should cost be compared across the County?

A city conversion and a rural detached unit may both be called an ADU, but they need very different work. Use the Seattle-area cost guide as a checklist, not a universal price.

County quotes should separate design, permits and fees. Add clearing, grading, foundation, construction, utilities or septic, access and drainage. Keep finishes, repairs and money for surprises in their own lines. Give every builder the same site facts and plans.

How should a regional builder be checked?

Washington L&I lets owners check the legal builder, registration, bond and insurance. Check the record again before signing. Request examples reviewed by the parcel’s own permit office, not just a broad Puget Sound portfolio.

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

Which County choices should be closed before design?

For land outside city limits, get written answers about urban-growth boundaries, lot size, critical areas, shorelines, septic and access. Describe the chosen form: inside the home, joined to it, a conversion or a separate building. Also record any current ADU. A general “ADUs are allowed” answer is not enough.

Also list each office that must review the project. Building, septic, water, fire access and road work may use different desks. Put each contact, filing and open condition in the schedule. One plan approval may not clear the whole project.