Well first you make a basement and then you put stairs in it where ever there is a space for example if you where trying to put stairs in your house to the basement you would have to have a huge amount of space and delete 6 squares of floor and but 2 stairs then in the basement you put the stairs where 6th squares are then you have basement stairs.
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use the sledgehammer tool and click the stairs
Unfortunately, you can't actually build stairs/upstairs for the sims on ps2 :'( get the sims 3 or the sims 2 (computer version(s)) to do this. Both work on pc, but only sims 3 works on mac :)
after loads of people claiming that you can't create one, You can, in fact, create a basement. go onto build mode and select basements. then, as if you were making a pool, click and drag the pointer to create your desired basement. then, place the stairs where you want them, and your ready to go.
go to the build mode and when you do the base of the house, there should be a small picture of stairs, if you click on that, it will bring up the stairs.
Yup, up to 5 floors!