a combination of pre - dimensioned and pre - engineered modular units which form at least a powder room within a room section of a building structure and a method of assembling same . the assemblage includes an interface unit having a drain - waste - vent piping system for the room , the unit being mounted to the surface of the &# 34 ; wet wall &# 34 ; of the room . the walls of the room may be either partially finished or completely finished at the time of assembly of the facility . the interface unit provides a unique template for locating the positions of , and for installing the components of the facility . by extending the powder room assemblage to include a bathtub and shower section , a unique full bathroom may be assembled in a very short time according to the method of the invention by persons of limited skills .