Septic soil in Avery County, NC, by the numbers
Very limited
In Avery County, NC, 94.2% of the surveyed land carries a "Very limited" rating for a standard septic drain 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. Avery County, NC is 247.338 square miles with a population of 17,811.
What Avery County's reading doesn't cover
USDA's survey covers the whole of Avery County, NC at once, not any single address. A local percolation test, typically $750-$1,850 (around $1,300 typical), is the only way to know what one lot in Avery County will actually do.
What a system would cost here
With 94.2% of Avery County rating "Very limited", a standard gravity system is not a safe assumption here. The engineered-system range, $10,000-$40,000 (mound or aerobic treatment), is the more realistic number to plan around, against $3,000-$8,000 for a conventional system on the rare lot that passes.
Avery County's rank among North Carolina counties
Nationally, Avery County, NC is the 758-most-constrained of 3,058 rated counties for septic soil. Within North Carolina alone it ranks 8 of 97.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems