"Do you want to go to the bathroom?"
Translation:Chceš jít na záchod?
Because that is the preposition that is used inn this case, that's all. Like the choices for in, at, to in English, one has to learn all the usage by heart. Jít na záchod means to go to use the toilet. Jít do koupelny means to go inside the bathroom (for whatever reason). Jít na koupelny does not exist. Unless it is a box and you can actually go on the top of the box.
Note that it is "DO koupelny" that is accepted. The preposition do takes the genitive case. Some information about prepositions and cases is available here: https://en.wikipedia.org/wiki/Czech_declension#Prepositions_with_certain_cases
Damnable language. JUST when you think you've got a handle on it, it throws you YET ANOTHER curve ball on the outside corner! Dekuji! Despite my stumbling around in the dark, I have to admit that in the course of the last 9 months starting from ZERO exposure to slavic languages, I actually HAVE learned (and retained) some things!