Did Rosemary's rape happen in the basement or in the Castevets' apartment?

I thought it was the apartment.

I assumed it was in the apartment. What made you think it was in the basement?

Early in the film, Rose hears chanting in a nearby apartment. So we know "the deed" is done nearby, not the basement.

I think it is definitely in the apartment. I love that apartment by the way. I know it’s a set, but I would have that in a heartbeat.. but without the satanic neighbours. I can’t have kids so they wouldn’t be interested in me anyway, lol.
I think I will watch this film right now xo

I thought it was in a chamber in the basement because the black yacht skipper in her dream is really the elevator man and he was telling her to go below deck meaning downstairs. Also Rosemary got spooked doing laundry in the basement as if her intuition was warning her of dangers to come.