Abstract: A building construction kit comprises a plurality of ground beams (10), for example of concrete, positionable and securable relative to one another to form a rigid foundation frame, a plurality of storey-height external wall panels (30) supportable on and securable to the beams of the foundation frame to form an external building shell and a plurality of floor units (70) which can be supported on and secured to the beams (10) or to the panels (30) to form a floor at, respectively, ground level or an upper level. Each external wall panel (30) comprises a galvanised steel frame (31) with a weatherproof cladding (35), for example of glass-reinforced concrete, at an intended outer side of the panel so that the shell formed by the panels provides a sturdy structure which is weatherproof without the need for a further weather-resistant finish.