How do I build the 2-story houses in the Sims 3 for Xbox 360?

I have the first floor already and a staircase, but how do I build the second floor? And this is for Xbox 360 ONLY. And another question, how can I view the first and second floor in buildings that have both when I'm in town? Because I went to the library and the gym and the only time I can view and use stuff on the second floor is when my sim goes up there by themselves.