What is a BBL number, and how do you look one up?
A DOB violation notice, an ACRIS deed, a property tax bill, a listing description: sooner or later one of them puts a ten-digit number in front of you labeled BBL, with no explanation of what it is or whether it even covers your exact apartment.
A BBL, short for Borough-Block-Lot, is the ID New York City files nearly every record for a property under instead of its address. Search a BBL directly and every record the city keeps under that number comes back together (taxes, violations, permits, deed history) rather than having to retype it into a different city office's own search box for each one.
Where the number comes from
A BBL is one ten-digit string built from a borough digit plus a tax block and lot: five digits for the block, four for the lot. New York City's Department of City Planning defines it as exactly that combination, unique citywide, and it is the format Geosupport, the citywide geocoding system every agency's software is built on, uses to answer "which parcel is this" for a tax bill, a deed, a permit, or a violation. An address can be spelled several ways and still mean the same building. A BBL, in the city's own systems, is not supposed to drift that way, though the block-and-lot number attached to a specific building has changed over the decades as lots were split and merged. An old violation can end up sitting under a number the same building no longer answers to.
A lot is not a building: the BIN
A BBL identifies a tax lot, not a building standing on it. A BIN (Building Identification Number) does that job instead, and the two numbers move independently: the same DCP Geosupport documentation notes that a lot can carry more than one building, or none at all, and that a BIN stays attached to its building even when the lot underneath it is later subdivided or merged into another BBL. A rowhouse with a garage on the same lot, or a lot awaiting construction, is where that distinction actually shows up.
It also cuts the other way. A lot with no building identified on it yet gets a placeholder BIN: the borough's code followed by zeros, pointing at no specific structure. The Department of Buildings still logs violations and permits against that placeholder for other buildings, so it is not simply left blank. A records search keyed strictly to a BIN can pull back a stranger's history on an unbuilt or unmatched lot instead of coming back empty. The BBL has no such gap: every taxed lot in the city has one, building or not.
When your own BBL isn't the one that matters
Most NYC lots carry one address and one BBL, and that's the end of it. Condos break the pattern. When a building converts to condominium ownership, each unit gets its own BBL for deed and closing purposes, while the building's property tax is still billed against one base lot for the whole structure: the "billing" BBL. Search a unit's own BBL for a record filed at the building level, and the record can simply be sitting under the billing BBL instead of the one you typed.
Across the roughly 1.62M addresses New York City's own address directory maps to a BBL, about 22k of them, spread across roughly 2,508 buildings, carry this exact split: an individual unit BBL that differs from the building's billing BBL. That's about 1.4% of all mapped addresses, concentrated entirely in condo buildings, and it's the practical reason a records search built around one lot number quietly misses another. Guide 05's violation guide covers the related case on the DOB side: the city has changed how it writes a lot number down over the decades and never restated the old violations to match, so a search built for one lot width can miss a legitimate record from before 2012 entirely.
Finding the number and using it
The Department of Finance publishes the citywide digital tax map every BBL appears on, the official public map the number was built for. It won't resolve a condo unit's billing lot for you, or tell you which of a building's violations were filed under an old number.
That's the part worth having a tool for rather than doing by hand. Search a BBL directly and Nosy Neighbor resolves it the way the records actually work, condo unit billing lot included, and returns taxes, violations, permits and deed history for the property in one report. An ACRIS deed and mortgage history is one of the records that comes back keyed to exactly this number, if you're trying to work out who actually sold a place and what they paid for it.