Skyway ADU Rules: King County’s Code, Not Renton’s

Skyway ADU rules come from King County, not the City of Renton, because Skyway-West Hill is unincorporated. The county’s zoning code allows two accessory dwelling units on an urban lot, each up to 1,000 square feet of heated space, asks for no added parking, and requires a notice recorded on your title before a permit is issued.

At The Rache Team we have guided families through Renton and the West Hill for more than 26 years, and the most common mistake we see with a backyard cottage plan in Skyway is starting from Renton’s rulebook. A Renton mailing address does not make a Renton permit. So on October 9, 2026 we read King County Code Title 21A in the county clerk’s copy updated September 9, 2026, along with the county permitting division’s March 2026 handout on accessory dwelling units. This guide reports what those two documents say, where the Skyway ADU rules differ from Renton’s, and how to check a specific lot.

Why Skyway ADU Rules Come From King County

Skyway-West Hill sits between Renton and Seattle and belongs to neither. It is an unincorporated part of King County, so the county writes its zoning and issues its building permits. The county’s own form says it plainly: the Permitting Division “provides land use and building permitting for the unincorporated areas of King County.” Our page on whether Skyway-West Hill is part of Renton covers what that means for taxes and services.

The county’s zoning code also gives the neighborhood its own chapter. King County Code chapter 21A.09D is titled “Skyway-West Hill Subarea Geography,” and it sets the density, setback and height figures for residential lots here. Where that chapter and the rest of the zoning title conflict, the Skyway chapter controls, with three exceptions: critical areas, shorelines and special overlays.

One caution before the details. The line between Skyway and the City of Renton runs through the West Hill, and some blocks on the south and east edges are inside the city. Earlington is one of them. A home there follows Renton’s code, which we covered in our guide to Renton homes with ADU potential. Always confirm the jurisdiction on the parcel record before you apply the Skyway ADU rules to a home.

What Do the Skyway ADU Rules Allow?

The standards live in one subsection, King County Code 21A.08.030.B.7.a. The county defines an accessory dwelling unit as a separate, complete dwelling unit attached to or contained within the primary home, or in a separate structure that is accessory to it. Here is what the subsection says for an urban lot such as one in Skyway.

Two Units Per Lot

For lots in the urban area, the code allows two accessory dwelling units per lot. The pair can be one attached and one detached, two attached, or two detached. Two detached units may share one building or sit in two.

Size: 1,000 Square Feet, With Three Exceptions

Each unit is limited to 1,000 square feet of heated floor area and 1,000 square feet of unheated floor area. Unheated space is the garage, storage or shop part of the building. The code then lists three exceptions:

• Basement or attic conversions. When the unit is wholly contained within a basement or attic of the main house, the size limit does not apply.
• Basements under a detached unit. Floor area in the basement of a detached unit does not count toward the maximum.
• Trading unheated space for living space. In the urban area, a unit that uses less than its unheated allowance can add that amount to its heated area, up to 1,500 square feet. The code’s own example is 1,250 square feet of living space paired with 750 square feet of unheated space.

Height and Setbacks

A unit that is not wholly inside an existing dwelling may not exceed the base height for its zone. The Skyway-West Hill table sets that base height at 35 feet in the R-4, R-6 and R-8 zones. The same table sets a 10-foot minimum street setback, a 20-foot setback for a garage, carport or fenced parking area measured along the driveway, and a 5-foot minimum interior setback.

The table also caps impervious surface, meaning roofs, driveways and other hard surfaces, at 55% of the lot in R-4, 70% in R-6 and 75% in R-8, with an adjustment for smaller lots. On a compact lot with a long driveway, that cap can bind before the setbacks do.

Garage Conversions

The code says an accessory dwelling unit may be converted from an existing structure, “including but not limited to garages, even if the existing structure is legally nonconforming with respect to setbacks or maximum impervious surface percentage.” In plain terms, an older garage that sits closer to the lot line than today’s rules allow can still become a unit, as long as it was built legally.

Parking, Streets and Attached Units

No additional off-street parking spaces are required, and no public street improvements are required. An attached unit must share at least one common wall with the main house and appear to be part of one structure. A breezeway only counts as a connection if it is under 10 feet long, shares a wall with both buildings, is fully enclosed and is heated.

Owner Occupancy

The subsection we read contains no requirement that the owner live on the property. That matches state law. RCW 36.70A.681 bars cities and counties from requiring owner occupancy for ADUs inside urban growth areas.

Skyway ADU Rules vs. Renton’s Rules

Both codes start from the same state law, so the headline is the same on each side of the city line: two units, no owner-occupancy rule. The details are where a plan drawn for one side fails on the other. The Renton figures below are from Renton Municipal Code 4-2-110C, which we read for our Renton ADU guide in September 2026.

• Size. Renton caps each unit at 1,000 square feet of gross floor area. King County counts heated and unheated space separately, and lets a unit in the urban area reach 1,500 square feet of heated space by giving up unheated space.
• Height. Renton limits a detached unit to 24 feet. In Skyway the limit is the zone’s base height, 35 feet in R-4, R-6 and R-8.
• Parking. Renton requires one space per unit unless the lot is within a half-mile walk of a major transit stop. King County requires no additional spaces anywhere.
• Setbacks. Renton’s detached units follow the zone’s front setback, a 5-foot side setback in its R-4, R-6 and R-8 zones and a 5-foot rear setback. Skyway’s table uses 10 feet from the street and 5 feet from interior lot lines.
• Recorded notice. King County requires a notice on title for every ADU. We did not find a matching requirement in the Renton section.
• Permit counter. Renton permits go through City Hall. Skyway permits go through King County Permitting.

The practical result is that the Skyway ADU rules are the looser set on size, height and parking. A two-story detached unit over a garage that would not fit under Renton’s 24-foot limit can fit under King County’s 35.

Do Skyway Lots Have Room for an ADU?

The Skyway ADU rules only matter if the lot can hold the building. When we measured King County’s parcel records for our guide to Renton lot sizes by neighborhood, the 4,126 single-family lots in Skyway-West Hill had a median size of 7,480 square feet, and 22% were 10,000 square feet or larger.

The larger lots are not where most buyers expect. The Campbell Hill and Langston side of the neighborhood had a median lot of 8,505 square feet. The Bryn Mawr and Lakeridge slope above the lake, where the views are, had a median of 7,200. So the blocks with the most yard for a detached unit tend to be the inland ones, and the view blocks tend to have less flat ground to work with. Our guide to Renton view homes shows where the county records those views.

Slope is the other limit. King County’s critical areas chapter, 21A.24, is one of the three things that override the Skyway chapter, and state law says its ADU protections do not apply on lots with critical areas or their buffers. A steep rear yard on the lake slope can be mapped as a hazard area even when the lot is large. Check the map before you count on the square footage.

Permits and Costs Under the Skyway ADU Rules

Three items in the Skyway ADU rules and the county’s handout affect your budget and timeline.

• The notice on title. The applicant must record a notice that identifies the unit as accessory, and it runs with the land. King County will not approve the construction permit until it has proof of recording. The county provides an affidavit form, and the recording fee can be added to your permit invoice. A future buyer of the home will see this notice in the title report.
• Sewer or septic sign-off. The county handout says an ADU may need a sewer certificate, public health approval, or both, reviewed the same way as any new home. Ask your sewer district early.
• School impact fees. King County Code 21A.43.070 excludes accessory dwelling units from school impact fees. Other permit fees still apply, and the county posts its current fee schedule on its permitting pages.

We are not quoting a total permit cost or a construction cost. Both depend on the design and the lot, and King County Permitting or a licensed builder is the right source for a number.

How to Check a Lot Against the Skyway ADU Rules

1. Confirm the jurisdiction. Look up the parcel in King County’s iMap or Parcel Viewer. If the jurisdiction reads unincorporated King County, the rules in this guide apply. If it reads Renton, they do not.
2. Find the zone. The same record shows the zoning. The height, setback and impervious surface figures above are for R-4, R-6 and R-8.
3. Check for critical areas. Turn on the steep slope, landslide and stream layers. A mapped area or buffer changes what can be built and where.
4. Measure what is already there. Add up the house, garage, driveway and patios against the impervious surface cap for the zone.
5. Pull the permit history. If the home already has a second kitchen or a finished garage, ask whether it was permitted as an accessory dwelling unit and whether a notice on title was recorded.
6. Read the title report. Private covenants and easements are separate from zoning and can limit a second building.
7. Call King County Permitting. The office is at 206-296-6600. Applications are filed online through MyBuildingPermit.com under King County, Building, Single Family Residential, New Construction, Accessory Dwelling Unit.

You can read the handout yourself on King County’s permit forms and resources page, and the full zoning title in the King County Code. For the neighborhood itself, see our Skyway-West Hill guide and our Skyway West Hill home prices snapshot.