A certificate of occupancy (CO) is a document that details what someone can legally use a property for, and the limits to which the property can be changed for that use. For example, a CO could state that a property can be used as an office, a private residence, or a restaurant. Many cities and towns will have online databases where you can download a copy of a property’s CO. Sometimes, it is possible to change a CO to cover a different use, but this can be costly and time-consuming.