Typically, when changing a house to commercial use, you should expect to rebuild any EXTERIOR entry stairs and/or porches used to access the commercial function. Existing houses usually have a change of level at the entry door threshold and entry stairs that are too steep or uneven to meet current commercial code.
You may need new/additional handrails at any INTERIOR stairs serving the commercial use, but the slope of interior stairs is generally exempt from review in a change of occupancy.
For the typical house, a single exit from the commercial area will usually be sufficient, even when it is the basement or second floor that will be used commercially. For large houses, the increased exit distance may mean a second exit.
For a basement to be converted to commercial use, the code requires 7' 6" of ceiling height (though there may be a few inches of wiggle room, via a simple appeal process). If the basement would be used only for storage, 7' 0" of headroom should be okay.