Even though one wouldn't guess so at first glance, the Faculty of Mathematics by Bevk Perović in Ljubljana (Slovenia) is actually an extension of an old edifice. On top of an existing, two story building, the new structure was placed - with a completely different programme. At a second glance, one could still see the old building shining through on the ground floor, retaining it's own identity whilst being absorbed into the new building.
Since the new building does not have a ground floor per se, the architects went for a different approach to create public spaces in the building: these large, transparent rooms are carved out of the building mass. By doing so, the second floor becomes what normally the ground floor would be: the public realm of the building. It consists of the lecture halls and the two larger communal areas of the faculty, with all the other programme of the school on the higher floors.
I found this through archdaily.com