BLDG. UNIT is a term used to refer to a unit in a building, such as an apartment or a senior living facility. An APT is an abbreviation for an apartment, which is a type of housing unit typically found in a building. A SEN is an abbreviation for a senior living facility, which is a type of housing unit designed for elderly people. Therefore, BLDG. UNIT can refer to either an apartment or a senior living facility.