01 · Challenge
Extra residential storey on a post-war staircase flat
A homeowners association in Rotterdam-Zuid wanted to extend the top floor of their five-storey stairwell flat by adding a storey. However, the existing structure from the 1950s was not calculated for an additional storey. Moreover, the homeowners association had to give formal approval and a building permit was required. The municipality of Rotterdam has an active policy for adding storeys but also sets requirements for architectural appearance and the visible roof profile.
02 · Our approach
Light steel frame, homeowners association supported
We designed an additional storey in a lightweight steel frame with prefabricated elements, minimising the placement weight and protecting the existing structural system. The structural calculation showed that the foundation piles could carry the extra load provided the steel frame was placed on the existing exterior facades. We guided the homeowners' association through the decision-making process, including obtaining the required majority in accordance with the deed of division. The building permit was submitted to the municipality of Rotterdam.
03 · Result
42 m² extra penthouse, building permit in 11 weeks
The building permit was granted within 11 weeks. The owners' association approved with a qualified majority. The additional storey produced a new penthouse apartment of 42 m² with an 18 m² roof garden at the rear. The residents of the floors below are barely aware of the construction, partly because the installation time was limited to six weeks thanks to the prefab approach.