Alleghany County, NC's soil, for a septic absorption field

Very limited

Soil across 94.7% of Alleghany County, NC rates "Very limited" in USDA's septic-absorption-field interpretation. Practically, that rules out a simple gravity system for most of the county - 94.7% of it sits in USDA's most-constrained bucket. Alleghany County covers 234.835 square miles and counts 11,379 residents.

This is a county figure, not a parcel figure

USDA's survey covers the whole of Alleghany County, NC at once, not any single address. A site-specific perc test, usually $750-$1,850, is what a health department will actually require before permitting a lot in Alleghany County.

System cost, conventional vs. engineered

Given Alleghany County's rating, plan for the engineered-system range rather than the conventional one. 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.

Where this sits nationally

Nationally, Alleghany County, NC is the 700-most-constrained of 3,058 rated counties for septic soil. Within North Carolina alone it ranks 7 of 97.

Further reading

How this rating is built, and its real limits · Conventional vs. engineered septic systems