Washington County, MN: the USDA septic-suitability picture
Very limited
Washington County, MN: USDA's soil survey calls 80.2% of the county "Very limited" for a septic leach field. A standard gravity system routinely fails on ground like this; the same share of the county falls in USDA's worst absorption-field bucket outright. The county spans 384.762 square miles, home to 283,960 people.
What this number does and doesn't tell you
Washington County's rating is an average across the whole county, not a promise about any one lot inside it. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.
Washington County: conventional vs. engineered cost
80.2% of Washington 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, Washington County, MN ranks 2,030 most-constrained nationally, and 72 of 86 within Minnesota.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems