Three rooftop additions expand the 1970 modern landmark designed by Josep Lluis Sert to accommodate growing departments of the History of Science, Statistics and Computer Services, and the world acclaimed Collection of Historical Scientific Instruments. The main addition at the East Terrace is a four-story structure replacing a one-story wing. Housing the Department of the History of Science and the Collection, this wing is configured as a four-story townhouse with offices and classrooms and a fully glazed internal stairway. The ground floor Collection, with its new location on the public level, and second-floor Temporary Exhibition and Archives together become a suite of spaces that makes an extraordinary collection available to the public. The design approach respects the integrity and original intentions of Sert`s building, while developing new and renovated space with a complimentary and boldly contemporary architectural language. The limitations of the existing structure required the new additions to be lightweight in concept and construction. The additions are minimal; steel-framed glass volumes are set into the sculptural mass of the original building like crystals embedded in rock. The cast-glass channel enclosure is luminous and light- a counterpart to the robust texture of the original precast concrete- and provides a density and rhythm compatible with the original building. The existing courtyard is transformed with a new dining terrace and green garden. The raised platform and plantings conceal air vents and equipment for the building`s mechanical room below. The new additions substantially contribute to making the full building meet current energy guidelines.