Rockland County, NY: the USDA septic-suitability picture
Very limited
Rockland County, NY: USDA's soil survey calls 75.7% of the county "Very limited" for a septic leach field. Expect the ground itself to work against a plain gravity system; USDA's strictest bucket alone covers 75.7% of the county. The county spans 173.685 square miles, home to 348,144 people.
What this number does and doesn't tell you
Two lots a mile apart in Rockland County can carry different soil, even though they share one countywide rating. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.
Rockland County: conventional vs. engineered cost
75.7% of Rockland 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, Rockland County, NY ranks 2,248 most-constrained nationally, and 51 of 61 within New York.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems