Since the 1970s there have been so-called OvSt's (short for "Ortsvermittlungsstelle", engl. 'local telephone exchange') spread throughout the city of Frankfurt. These OvSt's consist of prefabricated single story constructions.
The brief was to design a standarized addition of further levels to these buildings. Keeping in mind that the addition must also be realized among the buildings in the densely cramped inner-city areas. 
The design consists of a large bridge girder which is positioned above the longitudinal walls. This makes it possible to avoid extensive foundation works along these walls. The 50 metre long bridge girder supports two additional levels on top of the existing structure. This results in a space gain of 200%. To protect the steel girder from the varying weather conditions the steal structure was covered with an aluminium sun protection facade. The 'stacking' of the facade elements emphasize the structure with the colour scheme. The ridig sun protection elements fold upwards and form a horizontal sun protection for the offices on the third floor.

The design received the German Steel Construction prize (Deutscher Stahlbaupreis) in 1996.