What is an APN (Assessor's Parcel Number)?
An APN — Assessor's Parcel Number — is a unique numeric or alphanumeric code that a county assessor assigns to each parcel of land in their jurisdiction, used as the primary key across property tax records, deed filings, and GIS maps.
Think of an APN as a parcel's social security number: it never changes as long as the parcel exists in its current form, and it appears on every official document referencing that piece of land. Formats vary by county — some use dashes and dots (e.g., 123-456-78), others use pure numeric strings — but every parcel in every U.S. county has one.
The APN is the linking key that makes cross-database work possible. A deed filing references the APN; so does the tax roll, the assessor's value card, the GIS boundary file, and any permit the county issues for the property. If you want to join data from multiple county sources, the APN is the join key.
APNs are assigned when a parcel is created and retired or replaced when a parcel is split, merged, or replatted. A single large parcel split into two lots will generate two new APNs; the original is retired. This means historical APN lookups can be tricky: an APN from a deed five years ago may no longer exist in its original form.
When using Tracts data, the APN field is normalized and preserved exactly as the county provides it — no reformatting that would break joins against county-native datasets. It is also the recommended key for cross-referencing our parcel records against GIS boundary files for mapping or spatial analysis.