Abstract: A kit for the construction of a building comprises a series of modular floor units and an elongate aligning member. The modular units are assembled together in a horizontal plane to form a floor and comprise respective elongate border portions disposed end to end in the horizontal plane. Each border portion is formed with a first outer surface lying across the horizontal plane, a first longitudinal tongue substantially parallel to the first surface, and a first longitudinal groove formed between the first tongue and the first surface. The elongate aligning member is formed with a second surface lying across the horizontal plane, a second longitudinal tongue parallel to the second surface and to be received in the first groove, and a second longitudinal groove formed between the second tongue and the second surface to receive the first tongue to thereby align the series of modular floor units in the horizontal plane and prevent these floor units from moving with respect to each other in the horizontal plane.