While I don't know the specifics here, my understanding from being there a few years ago was that most lower-income workers in Singapore commute in from Malaysia.
Most of them live in migrant dormitories, or usually with the case of maids, accomodation provided by their employer. Some apartments will have “maids quarters”, which will be a seperate room with its own entrance/exit. But that’s not terribly common in Singapore. Typically they’d just have their own bedroom.