We (me and my parents) always say: "上厕所", and: "上卧室", ect. We (me and my parents) always say: "下厨房".
Personally my house has the kitchen at the top floor.
Why is that?
2 Answers
上 as a verb could mean "go to" (i.e. could be replaced with 去 without change in meanings) and "ascend/board" which could be understand as go up onto since the floor of vehicles are generally higher and require a upward movement. Anyway, in phrases such as 上卧室, 上 just means "go to". In 上厕所, 上 could mean "go to", but more often it contains the meaning of actually using the toilet. So it is possible for a person 去厕所 but not 上厕所, but most of the time such distinction wouldn't be made.
As for 下 as a verb, it means "go down to" physically or figuratively. In 下厨房, 下 contains a figurative meaning of "get down to the work", so if you are just getting something from the kitchen you wouldn't say 下厨房 and you could say 下厨房 for meaning of "go to the kitchen and cook".
上 and 下 are also used to distinct the start and end of certain activity, such as 上课、下课、上班、下班. In such scenarios, 上X could also be used for the time points during such activity.
Anyway, these two are pretty hard to master via a clear set of rules. You're just gonna have to get familiar with as many patterns as possible.
上 and 下, the original meaning is up and down, as adverbs. They are very frequently used as verb.
(1) The original meaning, for example, 上山砍柴，下河捞鱼。
(2) The social level, for example, 上得了厅堂，下得了厨房。
(3) The begin and end, for example, 上课，下课，上车，下车。
(4) The geometric sense as in map, for example, 北上，南下。
(5) To participate, for example, 教练让他上场，他受伤下来了（球场）。
(6) Sometimes 上 and 下 are used independently, not in the opposite meanings. for example, 这个贼被众人拿下了（a final ending）。我上不了手啊（can not help and participate）。你叫我打她，我下不了手啊（can not do it）。
(7) As in slang, 大哥，咱们今天就下手吧。
(8) "上 for going to somewhere" has a meaning of in a hurry. 上厕所，上医院，should be this type.
(9) As a normal verb meaning "go", 上 is used only, but not 下. for example, 你上哪儿去？上北京，上广州，上朋友家去，
上卧室 is case (9), and 下厨房 is case (2). They cannot be compared from the same angle.
go tohere, but why use上with厕所, and下with厨房is a challenging question...Here're two references: 上厕所下厨房的由来 为什么叫上厕所、下厨房