Toulouse architects BAST have renovated a derelict house in the French city by adding a corrugated steel extension that contrasts with the existing masonry (+ slideshow).

BAST responded to planning regulations outlawing the demolition of the existing house by designing a vertical extension that will give its inhabitants an additional storey once the interior refurbishment is completed.

The metal-clad addition replaces the building's damaged roof and sits on top of existing limewashed stone and brick walls, which echo the construction of other buildings on the street.

"We wanted to create a strong contrast between the part retained and the new part - to contrast massiveness of masonry against the abstract extension," architect Laurent Didier told Dezeen.

The angular structure features an offset gable and is punctuated by small windows on the south and west sides. The use of the strong but lightweight corrugated material reduces stresses on the lower storey.

"The extension allows the metal to not overload the existing foundations and walls," said Didier, adding that the weight of the new structure is equivalent to that of the old roof.

A row of roof lights along the north-facing surface brings a soft and consistent natural light into the upper floor of the building.

The ground floor will contain an open plan living room and kitchen, with a separate area housing a bedroom, bathroom and storage space.

A new framework constructed inside the existing walls will support a first floor containing two bedrooms, a bathroom and a mezzanine office.