Building in Skyway and White Center: What Unincorporated King County Owners Need to Know

Skyway, White Center, and the rest of North Highline sit right next to Seattle. Many residents have Seattle mailing addresses. But when it comes to building, these neighborhoods play by different rules, because they are unincorporated King County.

That difference trips people up all the time. We have met owners who spent months reading Seattle's new zoning rules, only to learn none of them applied to their lot. As a studio rooted in South Seattle and South King County, we want to change that. Here is what owners in these communities should know before they plan an addition, a backyard cottage, or new homes.

First, confirm where you are

A Seattle mailing address does not mean Seattle zoning. Look up your parcel in King County's iMap or Parcel Viewer to see your jurisdiction and zone. If your property is unincorporated, King County, not the City of Seattle, issues your permits, and your zoning comes from the King County Code, Title 21A.

Who issues your permits

King County's Department of Local Services handles permits in unincorporated areas. A few practical notes from the county's permit pages:

• Applications start in MyBuildingPermit.com. Once accepted, documents are uploaded through ProjectDox.
• The county posts a permit times dashboard showing turnaround for residential new construction and for additions and alterations. Check it before you set a schedule.
• Properties on septic need Public Health approval before applying, and flagged historic properties need Historic Preservation Program approval. Without them, your application is considered incomplete.
• From October 26 through November 6, permit customer service moves temporarily to the Renton Library (100 Mill Avenue S), Monday through Thursday, 10:30 am to 3:30 pm.

ADUs: read the county rules and the state law together

This is where we urge the most care.

King County's permit FAQ currently describes its ADU rules this way: one ADU per primary home, owner occupancy of either the ADU or the main home, a minimum lot size for detached ADUs (5,000 square feet in UR and R zones), and a general size cap of 1,000 square feet. No additional off-street parking is required, and owners must record an ADU Notice on Title.

State law, however, has moved. Under RCW 36.70A.680 and .681 (the law many people know as HB 1337), cities and counties planning under the Growth Management Act must allow two ADUs per lot in urban growth areas, cannot require owner occupancy, and cannot cap ADU size below 1,000 square feet. Counties apply these rules in their unincorporated urban areas, and MRSC notes that jurisdictions with June 30, 2026 update deadlines have until December 31, 2026 to comply.

What does that mean for you? The county page and the state law do not fully match, and that gap matters for design. Before you commit to a plan, ask the Permitting Division which rules apply to your parcel today.
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Middle housing works differently here

Washington's middle housing law, HB 1110, was written for cities. That is why Seattle now allows multiple homes on most residential lots. Unincorporated areas follow county zoning instead, so the number of homes you can build depends on your King County zone and its density rules. The county calculates density by multiplying lot area in acres by the zone's base density, then rounding.

There are signs the county is open to new housing types. Washington's co-living housing rules took effect January 1, 2026, and King County notes that state rules preempt its code until the code is updated. In Skyway-West Hill, the County Council considered a housing cooperation agreement with the King County Housing Authority for a community of about 30 manufactured homes affordable to households at or below 80% of area median income, with adjusted development standards.
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Community plans shape what comes next

Skyway-West Hill and North Highline both have subarea plans, first adopted in 2022. The county reports on performance measures every two years. These plans grew out of years of resident work, including anti-displacement strategies shaped by the communities themselves.

The Skyway Community Center is a good example of that persistence. Residents named a community center as a priority back in 2009. In April 2026, King County Executive Girmay Zahilay announced plans to buy the 3.82-acre Skyway Park Bowl site, which the Skyway Community Advisory Committee strongly recommended. Design focus groups and public meetings are planned through 2026, and advisory committee members are paid $75 per hour for their time. We love seeing residents compensated as experts in their own neighborhood.

Our advice for owners

Start with a parcel check

Confirm your jurisdiction, zone, utilities (including septic or sewer), and any critical areas before paying for design.

Ask the county early

A pre-application conversation can surface issues like setbacks, tree replacement, or recreation space requirements before they become redesigns.

Design for your family and your block

Many South King County owners build for multiple generations. Plan layouts, entries, and accessibility with that in mind.

Stay engaged

Subarea plans and code updates are shaped by the people who show up. Your experience as a builder or homeowner is valuable testimony.

Let's look at your lot together

La Union Studio works across Skyway, White Center, and South King County, and we know how different the county process can feel. Book a consultation and we will help you confirm what your property allows and map a realistic path to building.

FAQ

Do Seattle's zoning rules apply in Skyway and White Center? No. Skyway, White Center, and North Highline are unincorporated King County. King County issues permits there, and zoning comes from the King County Code, Title 21A.

How many ADUs can I build in unincorporated King County? King County's FAQ currently lists one ADU per home, but state law requires counties to allow two ADUs per lot in urban growth areas. Confirm the current rules for your parcel with King County Permitting.

Where do I apply for a building permit in unincorporated King County? Applications start at MyBuildingPermit.com, and accepted applications move to ProjectDox for document upload. King County's Department of Local Services reviews them.

Sources

King County Permits: Frequently Asked Questions · King County Permits: What's New · MRSC: Accessory Dwelling Units · King County: Subarea Plans · King County Executive: Skyway Park Bowl announcement (Apr 22, 2026) · King County Council: Proposed Ordinance 2026-0014