I believe they have a department for this and I did some research and found out that planning on a new physics building is currently being done (seems my information was outdated). The "Bauherr" ist the senate of the city, so at least in this case they are not doing it "in house".
Source: https://www.tu.berlin/facilities/services/baumassnahmen/ersatzneubau-experimental-physik-ex-p
Dann wären die Folgejahre aber hart verwirrend