Building permits, zoning and septic rules in unincorporated King County, Washington

Yes. Unincorporated King County requires a building permit for construction, enlargement, alteration, repair, moving, demolition and occupancy changes¹. Some common property improvements are exempt, but the county should confirm the specific project before work begins¹. Applications deemed complete on or after March 15, 2024 use the 2021 Washington State Building Code with state and King County amendments¹¹. King County requires a pre-construction conference for all new construction, inspections at project stages and visible work until approval; the permit holder must request each required inspection⁴. An owner working on their own property may be exempt from contractor registration, but not when building to sell, demolish or lease, and that exemption does not remove building, electrical, plumbing or land-use permit requirements⁶.

King County Permitting issues building and land-use permits for unincorporated King County; each incorporated city administers its own zoning, land-use and building-permit rules². Septic permits throughout King County remain with Public Health - Seattle & King County³. Call King County Permitting at 206-296-6600; phone service is available from 9 to 11 am and 12:30 to 2:30 pm on Monday, Tuesday, Thursday and Friday¹². The office is at 919 SW Grady Way, Suite 300, Renton, WA 98057¹². Most building and land-use applications can be filed through MyBuildingPermit.com, while some use email or another portal; online services include permit searches, status checks, fee payment and inspection scheduling¹. The 2026 fee schedule runs through December 31, 2026, and most applications carry a $126 screening fee at submission¹³.

Yes. King County administers zoning in unincorporated areas, while each incorporated city administers its own zoning, land-use and permit rules². In unincorporated King County, setbacks and building height vary by zoning district, and overlays or parcel-specific conditions can modify the base district^7,14. Construction lots generally cannot be smaller than 2,500 square feet in UR and R zones or 5,000 square feet in A, F and RA zones, subject to the stated exceptions and nonconforming-lot rules⁷. Lot creation and lot-line changes must also follow the county subdivision rules and zone-specific density provisions⁷. Most properties can support one accessory dwelling unit per primary detached dwelling or townhouse, but either the ADU or primary home must be owner-occupied¹⁴. A detached ADU requires a primary dwelling, the applicable zone minimum lot size and a notice filed on title before permitting, and is generally limited to 1,000 square feet of heated floor area¹⁴.

Public Health - Seattle & King County administers on-site sewage permitting throughout King County³. A new system requires a site application, design permit and installation permit, and construction cannot begin until the installation permit is posted on the property⁵. Call Environmental Health Services at 206-477-8050; the septic technical voicemail is 206-477-8177⁵. State rules also require a permit before installation, repair, modification, connection or expansion except for a minor repair; the local health officer responds within 30 days of a complete application, and a permit may last no more than five years¹⁵. A failed system within a sewer utility's service area must connect when the utility permits it and adequate sewer service is available within 200 feet along the usual or most feasible route¹⁶. Only about 0.5% of King County land is rated without severe limitations for a conventional drain field, so make septic feasibility an early purchase contingency¹⁰.

Unincorporated King County allows overnight RV camping only in an approved campground; the county defines an RV as temporary living quarters for recreational camping, travel or seasonal use rather than a permanent dwelling⁷. An approved RV park limits a stay to 180 days during a 365-day period and must use a sewage system approved by Public Health - Seattle & King County⁷. A tiny house may be built on wheels and may contain up to 400 square feet, but it must comply with the Washington State Building Code and still needs local approval for its foundation, installation and location⁸. The RV-park rule does not itself say whether a single RV may serve as a long-term residence on private land outside a park, and King County's complete tiny-home occupancy rule also remains unconfirmed, so ask King County Permitting to review the parcel, proposed structure and intended use before purchase¹². Zoning setbacks and height limits depend on the parcel's district^7,14.

1. Confirm that the parcel is unincorporated and identify its zoning district, buildable lot status, setbacks and height limit^2,7,14.
2. Establish water: a home cannot be built or expanded without a source approved by Public Health - Seattle & King County or the state Office of Drinking Water¹⁷. A new or replacement private well serving one home and up to one ADU requires a county application through a certified septic designer or water-system engineer⁹.
3. Have the septic designer submit the site application and secure design and installation permits before construction⁵.
4. Submit building and land-use applications online where available and budget for the applicable fee guide plus the $126 screening fee charged on most applications^1,13.
5. For flood-hazard construction, submit the required elevation certificate after the lowest floor is placed and before further vertical construction¹⁸.
6. Request the pre-construction conference and each project-stage inspection, keeping work visible until approval⁴.

Start with the usable building envelope, not total acreage. Only about 45.2% of King County land has a slope of 15% or less, while 83.1% is wooded, so clearing, access and grading can narrow the practical homesite¹⁰. Only about 0.5% of the county is rated without severe limits for a conventional septic drain field, making septic feasibility an early due-diligence item¹⁰. Confirm the zoning district, legal lot status, subdivision history and any overlays before assuming acreage can be divided or built upon^7,14. Verify an approved water source, and if a private well is planned, account for county site review, water-quality documentation and pump testing^9,17. Flood-hazard projects require a lowest-floor-elevation inspection and an elevation certificate before further vertical construction^4,18. Budget around a 0.8% effective property-tax rate and a median annual bill near $6,785¹⁰.