Septic suitability in Thurston County, WA
Very limited
USDA puts Thurston County, WA at "Very limited" for septic absorption fields, the dominant reading across 91.6% of the county's soil. That reading means a conventional system is a poor bet here - 91.6% of Thurston County lands in the strictest "very limited" bucket USDA uses. The county spans 722.513 square miles, home to 302,912 people.
What this number does and doesn't tell you
Nothing about Thurston County, WA's SSURGO figure applies to a specific address - it's a county-wide average. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.
Thurston County: conventional vs. engineered cost
91.6% of Thurston County carrying a "Very limited" reading changes the realistic budget here. Conventional systems run $3,000-$8,000 where they pass; where they don't, engineered alternatives (mound or aerobic treatment, whichever the health department requires) run $10,000-$40,000 - budget toward the higher figure until a perc test says otherwise.
Where this sits nationally
Among the 3,058 rated counties on this site, Thurston County, WA ranks 1,088 most-constrained nationally, and 19 of 39 within Washington.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems