Perimeter blocks characterize Zurich, and one can associate the upcoming of providing affordable housing in the city with this system. These closed typologies are rooted in the BZO in chapter E (Quartiererhaltungszonen) although many city blocks are not entirely closed – leaving gaps.

These gaps generally stretch across two plot cells and portray an underexplored potential in the city. Though it is not profitable for owners to add a building on the too narrow leftover ground on their own plots, when two owners collaborate, they find space for new apartment buildings in the city centre. When building jointly, owners can add fillers that preserve existing buildings, create more apartments, and supply energy and heat with modern building systems - benefitting old and new buildings mutually.