Certificate of Occupancy

A certificate of occupancy (C of O) is the Department of Buildings document stating what a building may legally be used for and how many people or dwelling units it may contain, issued when construction or a major alteration is completed in conformity with the approved plans and applicable laws. It is the legal identity of the building: a structure being used outside its certificate — more units, a different use — is in violation regardless of how long the arrangement has existed.

Older buildings may lawfully lack a certificate if they predate the requirement and have not since been altered in ways that trigger one; establishing legal use for such buildings relies on other records. In diligence, comparing actual use to the certificate is one of the fastest ways to surface legalization risk.