Abstract: A modular building system including modules for a majority of structural elements making up a modem building. Modules include, floors, walls, elevator shafts, stairways, conduit assemblies. Facilities located on or off site are utilized to prefabricate most of the structural elements needed to construct a building. Finished floor modules are complete sections of floor including floor core, floor covering, insulation, ceiling beams and ceiling below. Wall modules include wall core, exterior wall covering, utility runs, wall beams and interior wall covering. Elevator modules include stackable elevator shells and guide tracks. Stairway modules include stackable stairway shells that may be stacked as many number of floors as needed. Utility conduit assemblies include all related utility meter gear and cabling, plumbing, etc. for gas, water, electricity, telecommunications, etc. services.