A three story rooftop extension was added to an existing Wilhelminian-times house. Plans of the four new spacious apartments - one of them a maisonette – were developed from a complex set of geometric reference grids, which were derived from the shape of the irregular 5-street intersection of Operngasse and Margeretenstraße. In section, these conflicting reference frames were merged into a unified structure by a meandering 3-dimensional ribbon, that eventually transitions into the topcornice of the existing facade, thus merging old and new. Architectural volume, glass facades and terraces orchestrate exciting views into the streets of Vienna and along its roofscape, creating a generous open atmosphere that reflects the urbanistic conditions of this premium location.