A villa in Leuven, Belgium, has an unusual new roof over its garden extension. Instead of being flat on the underside, the steel structure varies in thickness from about 2 inches to 18 inches (with an 8-inch layer of insulation on top). As the ceiling dips, the floor drops: The living area is four steps below the rest of the room. Fix lights from Lucide hang from above. The Highlands sofa is by Patricia Urquiola.