ZIP 99128 falls in Whitman County, Washington, where we hold 4,266 parcels measured as septic county-wide and 9,252 on sewer. We do not have a per-ZIP parcel count for this county, so this is the county total, not a count specific to ZIP 99128.
ZCTA 99128 is not entirely inside one county. Land-area share is from the 2020 Census ZCTA-to-county relationship file.
| County | Land in this ZIP | Documents held | On septic (county) | On sewer (county) |
|---|---|---|---|---|
| Whitman County, Washington | 93% | 49 | 4,266 | 9,252 |
| Latah County, Idaho | 7% | — | 4,971 | 6,520 |
Land-area overlap is from the 2020 Census Bureau ZCTA5-to-county relationship file, not an estimate of ours. Parcel and document counts are rows we hold, not a survey of every property in the ZIP. Last verified 2026-09-01.
We hold 4,266 parcels measured as septic for Whitman County, Washington as a whole, not broken out by ZIP.
ZIP 99128 spans 2 counties: Whitman County, Washington, Latah County, Idaho.