Abstract: A mobile building construction comprises a wall structure (1) intended, together with a roof, to define at least one room. The wall structure (1) comprises two spaced-apart shells (2,3) defining a gap-like space (4) which, at the top, has openings for filling heavy bulk material (13) such as macadam, gravel or the like, into said space so as to form a safety wall which is resistant especially to firing or explosive action. At the bottom, the space has one or more outlet openings (5) adapted, when desired, to allow emptying the bulk material (13) from the gap space (4) in order to release the wall structure from the load of bulk material, whereby to facilitate transportation of the building construction from one ground to another.