German term (edited): Umlageflächen

The "Umlageflächen" appear to be the same as the so-called "Gemeinschaftsflächen" ("common areas", such as corridors, staircases or reception areas), which are shared by the parties renting the office premises. For the purpose of determining the floor space rented by each party, these areas are then accounted for on a prorated basis and added to the actual office space rented.

The dictionary reference cited below includes the following entries:

Gemeinschaftsflächen (z. B. in einem Bürohaus) = common area (the plural S is missing here)
Gemeinschaftsflächenumlage (vom Mieter zu zahlender Beitrag für gemeinschaftlich genutzte Flächen) = common area assessment