When Lord Marchmain returns to Brideshead, he is accommodated on the ground floor (in the "Chinese drawing room").
However, when Charles Ryder is being shown around the castle by a commanding officer some years later, the two walks up a flight of stairs to the second floor where they enter the (same) "Chinese drawing room", intact with all details.
When Charles returns to Brideshead during the war, he is shown the "Chinese Drawing Room," where Lord Marchmain died. A modern sprinkler head can be seen in the ceiling.