Billy Wilder, 1960. Jack Lemmon's lonely Manhattan bachelor and his bosses' borrowed flat. Best Picture.
From Wikipedia
The Apartment is a 1960 American romantic comedy film directed and produced by Billy Wilder from a screenplay he co-wrote with I. A. L. Diamond. Jack Lemmon stars as an insurance clerk who, in hopes of climbing the corporate ladder, allows his superiors to use his Upper West Side apartment to conduct their extramarital affairs. He becomes attracted to an elevator operator in his office building, unaware that she is having an affair with the head of personnel. Ray Walston and Edie Adams feature in supporting roles.