Cassia County's SSURGO septic rating
Very limited
Cassia County, ID: USDA's soil survey calls 63.5% of the county "Very limited" for a septic leach field. Practically, that rules out a simple gravity system for most of the county - 63.5% of it sits in USDA's most-constrained bucket. The county spans 2,565.61 square miles, home to 25,976 people.
What this number does and doesn't tell you
This is a countywide, area-weighted figure across every soil survey map unit in Cassia County, ID - never a reading for one specific parcel. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.
Cassia County: conventional vs. engineered cost
63.5% of Cassia 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, Cassia County, ID ranks 2,647 most-constrained nationally, and 41 of 44 within Idaho.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems