Detail: Le Corbusier dreamt of roof gardens removed from the noise of the road. You are planning an entire city with a second level at a height of 18 m. Why is a project like this being realized in Holland, where building is traditionally low-rise?Arets: First, we wished to achieve a certain density. Second, we believe the programme we are proposing for Groningen – and also for Hoofdorp – should be a hybrid one, in which we want to introduce as much housing into the city as possible. The greatest problem in such cases is usually that there is no satisfactory outdoor realm. We want to offer housing with external spaces large enough to allow play and other activities. In Hoofdorp, we are trying to build a new urban centre. There, of course, the main level is the public realm. Beneath this is a folded structure that forms a second public level – not just a place to park cars, but a space that is landscaped as well. (Wiel Arets was interviewed by Frank Kaltenbach.)