Soil and perc tests in King County, Washington

USDA rates most of the soil in King County Very limited for septic absorption fields — 99.9% of the rated acres. A licensed on-site sewage system designer must complete a soil and site evaluation. Washington does not use a percolation test. USDA NRCS Soil Data Access (SSURGO) · checked 2026-09-16

Very limited

USDA rating, 99.9% of rated acres

checked 2026-09-16

Slope

Most common limiting reason

checked 2026-09-16

Yes

Site evaluation required

checked 2026-09-23

What is a perc test and does King County require one?

Yes. A licensed on-site sewage system designer must complete a soil and site evaluation. Washington does not use a percolation test.

The designer records soil type, depth to water, and other site conditions on soil logs. King County requires at least four soil logs for a single-family lot.

King County Board of Health Code § 13.28.020, Design Support Materials · checked 2026-09-23

How USDA rates the ground in King County

Very limited across most of the county — 99.9% of the rated acres. A rating describes the ground, not a parcel. The county's site evaluation decides each lot.

Very limited — 1,106,030 acres — 99.9% of 1,106,961 rated acres.
Somewhat limited — 931 acres — 0.1% of 1,106,961 rated acres.
Not rated — 51,998 acres, excluded from the denominator — mostly urban land and water.

99.9% of 1,106,961 rated acres. 51,998 acres unrated. Spans 3 survey areas. The largest are WA634 (v27), WA633 (v21), WA775 (v9). Versions established 2025-08-28.

USDA NRCS Soil Data Access (SSURGO) · checked 2026-09-16

What limits absorption here

The reasons USDA gives for the rating, by the acres they affect. A component can carry several, so the acres overlap.

Slope — 895,151 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Slow water movement — 520,807 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Depth to saturated zone — 450,915 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Seepage, bottom layer — 336,680 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Filtering capacity — 246,366 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Depth to bedrock — 221,092 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Large stones — 93,105 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Flooding — 59,219 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Ponding — 45,220 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Depth to cemented pan — 39,365 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.
Subsidence — 16,059 acres. Reasons overlap — a component can carry several — so these acres exceed rated acres.

Ratings by soil map unit

The largest map units in the county, each with its rating. A soil survey names the ground. The county's evaluation of your lot is what a permit turns on.

AgC — Alderwood gravelly sandy loam, 8 to 15 percent slopes — 126,500 acres in this county — rated Very limited. Dominant component: Alderwood (85% of the map unit).
197 — Playco loamy sand, 30 to 65 percent slopes — 36,922 acres in this county — rated Very limited. Dominant component: Playco (100% of the map unit).
255 — Tokul gravelly medial loam, 8 to 15 percent slopes — 33,827 acres in this county — rated Very limited. Dominant component: Tokul (70% of the map unit).
AgB — Alderwood gravelly sandy loam, 0 to 8 percent slopes — 29,471 acres in this county — rated Very limited. Dominant component: Alderwood (85% of the map unit).
EvC — Everett very gravelly sandy loam, 8 to 15 percent slopes — 27,924 acres in this county — rated Very limited. Dominant component: Everett (80% of the map unit).
200 — Playco very gravelly loamy sand, tuff substratum, 30 to 65 percent slopes — 27,048 acres in this county — rated Very limited. Dominant component: Playco (100% of the map unit).
AkF — Alderwood and Kitsap soils, very steep — 27,027 acres in this county — rated Very limited. Dominant component: Alderwood (50% of the map unit).
152 — Nimue loamy sand, 30 to 65 percent slopes — 24,909 acres in this county — rated Very limited. Dominant component: Nimue (100% of the map unit).

USDA NRCS Soil Data Access (SSURGO) · checked 2026-09-16

Sources

Each figure on this page has a government source and a checked-on date.

Public Health – Seattle & King County — On-site sewage/septic system program · checked 2026-09-23
Public Health – Seattle & King County — About Environmental Health Services · checked 2026-09-23
King County Board of Health Code §§ 13.20.030, 13.20.035, 13.68 · checked 2026-09-23
Public Health – Seattle & King County — OSS Permitting, Certifications, and Drinking Water Fees, effective January 1, 2026 · checked 2026-09-23
Public Health – Seattle & King County — Search on-site sewage/septic system records · checked 2026-09-23
Washington Administrative Code § 246-272A-0232 · checked 2026-09-23
Washington Administrative Code § 246-272A-0210 · checked 2026-09-23
USDA NRCS Soil Data Access (SSURGO) · checked 2026-09-16
King County — Septic System Maintenance and Side Sewer Repair Rebate Program · checked 2026-09-23

Verify before you act. Septic rules are set locally and they change. Check anything here before you spend money on it. Call the Public Health – Seattle & King County, On-Site Sewage/Septic System Program, on 206-477-8050.