This private house in the centre of Brussels has been completely transformed by the addition of a steel extension at the rear. This 6 m high extension standing 4.5 m long and 4 m wide and weighing around 4,500 kg, was designed as a whole and includes an integral staircase. As the new building section had been prefabricated in the workshop (a process that took three weeks plus a further one week for painting) it was installed in just three hours. The pared-down construction of 5 mm sheet steel reinforced with a set of five 20 mm thick by 100 mm wide portals made it possible to optimise the additional space, and the painted steel serves not only as a visual signature of the project, but also as an inner shell. The waterproof steel cocoon also acts as a natural vapour barrier beneath the external wall insulation around it. The steel is also fully recyclable.