BBL

A BBL is the identifier New York City assigns to every tax lot: a single digit for the borough, followed by the block number and the lot number within that block. It is the primary key of NYC property records — assessments, permits, violations, and recorded documents all attach to a BBL, which makes it far more reliable than a street address for tracing a property through municipal records. Addresses are ambiguous (corner buildings carry several, and vanity addresses abound); a BBL is not.

BBLs change only through real events — lot mergers, subdivisions, condominium formation — so a BBL is also a stable handle over time. Condominiums complicate the picture slightly: the underlying land keeps a base lot while individual units carry their own lot numbers in a high range, which is why unit-level and building-level records can sit under different BBLs.