No, in Dutch we're not afraid to actually say toilet or wc, if somebody talks about the badkamer they actually mean the thing with een bad, een douce en een wc (a bath, a shower and a toilet), not a substitute for toilet. If somebody would say Mag ik de badkamer gebruiken? I would expect them to take a shower.
In The Netherlands, the toilet and the rest of the bathroom are separated so Susande is right
For most parts of the Netherlands there's a seperate toilet and a toilet in the bathroom
The same is true here in the UK - we don't ask to use the bathroom when we need to use the toilet. Neither do we use "little boy's/girls room".
That's not to say we don't use euphemisms ("convenience" is one, while "to powder my nose" is used to mean "going to (use) the toilet") but most people just say toilet in all but the most polite of circumstances.
The only time we'd generally say "bathroom" here is when asking where it is in someone else's house, since the bathroom is usually where the toilet is. We'd still say "Can I use your toilet?" or something to that effect to ask permission.
ah, so is "need" (as translated) means something like "to use" rather than something like "i need to build or add a bathroom onto the house"?
I'd say Ik heb een badkamer nodig would normally mean you need to build this room onto the house. It could also mean you need to take a shower, but it would be an odd way of saying it, something like ik moet douchen (I have to take a shower) sounds more natural.
I go along with your translation. "I need a bathroom" denotes urgency where as "I need to use the bathroom" means you would like to as soon as its convenient.
The emphasis in the word ''badkamer '' is wrong. should be on the first syllable.
I see you are level 14 in German so presumably the concept of separable verbs isn't alien to you. I don't know if this is technically a separable verb, but it is certainly analogous, and nodig hebben can certainly be thought of as a single unit. If it is the logic of that combination that you are having trouble with then try thinking of it as "have need of" rather than simply "need".