banĉambro

Head-final, and both halves are already yours. Ĉambro is the head - a banĉambro is a kind of room - and ban-, the root of bani (to bathe), says which kind. The linking -o- drops because banĉambro is comfortable to say without it, though banoĉambro is not wrong. Compound words covers the rule; this word is about as clean an example as the language offers.

The fence with banejo. Both get translated "bathroom" and they are not the same word. Banejo is bani plus -ej-, the place suffix, and -ej- is broad: any place for bathing. That takes in the lido, the public baths, the swimming spot on the river - and, indoors, the bathroom too. Banĉambro names the ROOM and only the room, because ĉambro is baked into it. So every banĉambro is a banejo, and most banejoj are not banĉambroj. When an estate agent lists a flat, banĉambro is the word that cannot be misread.

And the room next door. Continental Europe often puts the bath and the lavatory in separate rooms, and Esperanto follows the continent: necesejo is the toilet, a different room with a different name. If you want the tub itself it is banujo or bankuvo, and if you want the act, bani sin - to bathe oneself, with the reflexive pronoun doing exactly what it does in French.