Abstract: A prefabricated room module (1) is provided for use in the construction of a modular building. The room module comprises a floor slab (2) of generally rectangular shape in plan, and load bearing walls (3,4) formed at adjacent sides of the slab and mutually adjoining at a common corner (5). The module has no ceiling slab opposite the floor slab (2) and an open face opposite one of the load bearing walls (4) which constitutes a party wall. Opposite another of the load bearing walls (3) the module may be open or include a further wall (7) which can have an access opening (10) therein for providing service access to a kitchen or bathroom pod (9) installed in a corner of the module. Edges of the floor slab (2) have projecting tongues (11) and fastening means for engaging complementary recesses and fastening means respectively of an adjacent module. The module may have a ceiling slab instead of a floor slab (2).