The building's listed status makes the required energy-efficient renovation a particularly challenging task. This is because changes to the appearance were not allowed. In order to preserve the look of the building, the decision was made to replace the units. Several test windows in different colours were submitted to the local authority. Not only was the appearance of the frames extremely important, but also the colour of the glass, because modern insulating glass is a different shade to the transparent, thin glass of the 1960s. In the end, for this energy-efficient renovation all the windows in the building were replaced with highly thermally insulated Schüco windows, improving the U value and thereby the energy efficiency of the windows to a great extent.