Bruno's father's office is unlike the rest of the house (except for the large bathroom near his parents' room) because it isn't a place where the children are allowed; it's not meant for family life. Instead, it's a place that's purely for work. Because of this, it intrigues Bruno in a way the rest of the house doesn't.
The office faces the dining room and the door is usually closed. When Bruno first mentions the office, he capitalizes his description of it, saying it is "Out Of Bounds At All Times." This means that his father's rule to not enter the office was given and reinforced very seriously.
The rest of the house is a place for fun. Bruno thinks of sliding down the banister from the top floor. His parents's room is on the floor below the top floor, and he thinks that he isn't supposed to be in their bathroom. The next floor is his room, Gretel's room, and the smaller bathroom. His favorite thing about the house is the banister.
The office in their old house had the same restrictions as the office in the new house. He thinks the rule about not going into his father's office is one of the most important rules he's ever learned.
Bruno's father spends a lot of the time he isn't in the camp in his office.