Construction systems for erecting building structures comprise a plurality of prefabricated interconnectable modular building units, each unit not meeting at least one of the ISO certification criteria for transport of cargo, but each unit comprising a frame shaped as a rectangular parallelopiped and comprised of framing members and a plurality of nodes, each node situated at a corner of said frame for selective interconnection with other units, the nodes and the exterior dimensions of the frame conforming to ISO shipping standards such that each unit is transportable using the ISO intermodal transportation system, and such that when the units are aggregated horizontally and vertically and adjacent units are interconnected, a building structure comprising at least one habitable space is formed. The modular units are assembled in a factory remote from the job site, and are there constructed to a semi-finished state, including installation of one or more of interior fit-out systems and finishes, exterior envelope systems, plumbing systems, electrical systems, environmental systems, and fire protection systems, following which the semi-finished modular units are transported from the factory to the job site, where they are craned into place and secured to form the structure being erected, a plurality of adjacent pairs of semi-finished modular units also being “stitched” together, and the semi-finished modular units are thereafter constructed to a finished state.