Can I Use The Space Under The Stairway?
Most of the time you can
use the space under your stairway for storage and if you can I suggest
that you do. Certain types and designs of stairs make it difficult to
build a storage area underneath the stairs, but if you can, just think
of all the things you could store there.
Most building codes requires that you use one hour fire retardant building materials, underneath any stairways that are going to be exposed or used for storage. You can accomplish this most of the time by using Type - X 5/8" drywall. Make sure that you understand how to finish the drywall, because most building codes also require a special type of drywall tape.
Most builders take advantage of any space they can underneath the stairways, if possible.
Special Note: I haven't ran into any minimum sizes for storage underneath a stairway.