For the riser? Or for the actual stairs going down the the basement?
In my area, there is little in the building code that stipulates stairs with less than a couple treads, so pretty much anything within reason goes. Over 3 or 4 treads and there are a lot more requirements (hand rails, minimum width, etc.) I do not think 24" would be wide enough for a full flight of stairs where I live.
If it is just one step on the riser, I think that would be fine.
-Suntan