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Explanation: My initial thought was "Endsumme" but then I thought what's this got to do with buildings or rooms?
"grand" can also mean "großartig", "prachtvoll", which might be in connection with a building or a room.
"Grand Total" can also refer to a "Gesamtheit", i.e. die Gesamtheit der Räume/Gebäude etc., although I have to admit I'm only guessing.