|This article is about an older version of DF.|
An office (called a study in some game-menus) is a room required by certain nobles. Some higher-ranking nobles require a "throne room", but this is the same thing, just fancier. These can simply be designated from a throne or chair using , and do not (necessarily) have to be a separate enclosed space separated from other areas by walls and a door.
If you assign an office to a dwarf, sometimes he will eat a meal in his office and complain about the lack of tables. You may wish to assign that dwarf his own dining room as well to avoid this thought. Alternatively just put a table next to the chair in said office.
If you put a table next to the office chair and assign it to be a dining room, both rooms will be used like they are supposed to. However, overlapping rooms drop considerably in value rating, making it harder to meet demands by higher ranking nobles. While this can be countered with high-value furniture (very helpful: artifacts), actual individual areas, designated separately from other areas (whether or not separated by walls) are the easier way to meet demands.