Basement should mean porão in Portuguese, but, in certain situations, it doesn't. Just today I was taking a police report. The female cop said she went to the basement of the nightclub in order to get to the toilet or something to that extent. Well, if there is a toilet there, and the environment is a nightclub, that place, where the toilet is, must not be accessible through only one door, and it must not be a place where people usually don't go. Instead, it must be a level like any other in the nightclub. In this case, we say parte de baixo (bottom part) in Portuguese or andar abaixo do térreo (level below the ground level).
Attic attracts the same issues: It should be sótão, but that is if we rarely access it or if we are not supposed to access it, so say it is where we keep our wine bottles in order for the wine to age. If we have an entire bedroom there and it is accessible by more ways than one door, then it should probably be called parte de cima (upper part).