A lightweight timber framed extension to a large Georgian single family house in the Forest Hill Conservation area of South London. The clients, a couple and their three children, were keen to improve the functionality of the house by providing a bathroom for the children on the top floor. The existing rear extension had already been built up from 2 to 3 stories and a further storey in brick and slate would have required substantial underpinning. The proposal was further complicated by the discovery of a stream running under made up ground that carried the foundations of the house. The solution was to keep the new structure to under 10% of the existing weight of the rear extension. This was achieved by building in a lightweight timber frame with rigid insulation between and over the frame and cladding in Anthrazinc, a dark grey standing seam zinc cladding system. The extension is deliberately simple in form, defering to the proportions of the Georgian house.