Mine was built-in during construction.
Coupla photos may answer whatever questions you may have.
The long room on the right is a dirty room (welder, buffer, grinder, etc)
The square room on the left is bathroom w shower.
Between the two rooms is an 8 foot wide pass through to smaller room.
Photos 4 and 5 show longer, cantilevered joists for landing to attach staircase.