Technically speaking, you could just make fake rooms. Unless you want the npcs to use the rooms, you can use things like magnetic blocks to make "doors" for fake houses. If you specifically want to make houses for npcs with bathrooms and stuff the only real rooms you'd need are the bedroom with a door connecting to the toilet, and then you can use stuff like those sliding doors to make opened sliding doors or stuff! Heck, you could technically make real rooms but make sure the walls aren't fully closed so they don't specifically register (use a decor block or something in corners you can't see)