Buying Land in King County, Washington

Rural Area zoning: genuinely low density, with real environmental limits attached

Outside King County's Urban Growth Area, the Rural Area designation covers most of the county's unincorporated land, and its zoning districts are set deliberately low-density: RA-10 (one unit per 10 acres), RA-5 (one unit per 5 acres, minimum lot 217,800 sq ft), and RA-2.5 (one unit per 2.5 acres). These aren't nominal minimums that get regularly varianced away -- King County's critical-areas regulations layer additional, genuinely enforced limits on top, including capping impervious surface in Rural Area zones at 8% of the lot (or 15,000 sq ft, whichever is less), specifically to protect groundwater recharge and limit stormwater runoff into salmon-bearing streams.

For a buyer, confirming the specific Rural Area sub-designation on a target parcel -- RA-2.5, RA-5, or RA-10 -- matters more here than in most rural-zoned counties, since the density difference between the tiers is substantial and directly determines subdivision potential, buildable area given the impervious-surface cap, and ADU eligibility.

The TDR Program: a real market, not a regulatory abstraction

King County's Transfer of Development Rights Program has operated since 1998 as a voluntary, market-driven alternative (or complement) to pure zoning restriction: a landowner on a 'sending site' -- typically rural, forest, or agricultural land the county wants to keep undeveloped -- can separate the right to develop their land from the rest of their property rights and sell that right as a tradable commodity to a developer building at higher density on a 'receiving site' inside the urban area. As of a May 2025 program update, King County's TDR Program had protected more than 148,000 acres of rural and resource land and open space since its founding. Parcels in rural forest focus areas, agricultural lands, or certain small nonconforming lots can qualify for 'bonus' TDRs, and enrolling typically requires either a Forest Stewardship Plan or a Farm Plan, with the sending property then permanently conserved through a recorded conservation easement.

This program is worth investigating directly for two different reasons depending on your position: if you own or are buying rural King County land as a long-term holding rather than a development project, enrolling as a TDR sending site can generate real income while formalizing (and typically improving the value case for) land you weren't planning to develop anyway. If you're evaluating a smaller rural parcel specifically for its ADU or building potential, purchasing a TDR from the county's TDR bank or an existing sending-site landowner can be the difference between qualifying for a detached ADU on a 2.5-acre RA-5 lot and not qualifying at all. See the dedicated conservation-easements page below for how the sending/receiving mechanics, pricing, and easement requirements actually work.

Septic through a recently updated county health code

Where a King County parcel isn't served by public sewer, on-site sewage systems are permitted through Public Health -- Seattle & King County. An on-site sewage system designer submits a site application, and once approved, a certified installer completes the work; construction can't begin until the installation permit is physically posted on the property. The county's on-site sewage code (Title 13 of the Board of Health code) was revised effective April 1, 2025 -- confirm current standards directly with the department for any parcel evaluation, since recently updated code provisions may not yet be reflected in older third-party summaries.