By 1960, UNESCO realized it would need more office space and asked Breuer, Bernard Zehrfuss and Pier Luigi Nervi to design a fourth building for the site along with an underground parking garage. Breuer envisioned a building with a modular concrete façade raised on as few columns as possible so as have a minimal impact on the visual effect of the open plaza. He and Nervi developed the design in great detail, but the building was never built. The two had a falling out with Zehrfuss in 1962 when Zehrfuss submitted an independent design for the project. A version of that design was built in 1965. The most striking feature of Zehrfuss's below-ground expansion was the sunken patios providing light and access to greenery.