Basement can refer to the following:
The basement of Henri Dijon's mansion in The Colonel's Bequest. Jeeves has a room in one section of the basement. The rest of the basement is sealed off, and only accessible through a secret tunnel under the Hedge Garden.
The basement of the Leyendecker Museum. Here can be found the Alcoholic Preservation Lab, Wolf Heimlich's office, the Mammology Lab, and Ernie Leach's office.