A lot of the questions I receive start off with “Where does it say that I can/can’t do XYZ?” Most of us know that stored materials don’t belong in stairwells, including the space under the stairs. But with square footage at a premium, we are sometimes asked to “prove it” which means finding a code reference to point to. This is where I can (usually) help save you some time.
For a question that involves the ongoing maintenance of a building rather than how a building is constructed, I would typically refer to the fire code that has been adopted in the jurisdiction where the building is located. The most common model codes used in the US are the International Fire Code (IFC) and NFPA 1: Fire Code. NFPA 1 references NFPA 101 for means of egress requirements, so I would go directly to NFPA 101: Life Safety Code. Note that state and local requirements may vary.
So…where does it say that you can’t use the space under the stairs (or on the stairs) for storage (or anything else)? In the IFC (2015 edition) I would reference paragraph 1023.1 which says, “An interior exit stairway or ramp shall not be used for any purpose other than as a means of egress and a circulation path.” In the IFC Commentary for this paragraph, it uses storage in stairways as an example of a use that might obstruct the path of exit travel and is therefore not allowed. If the stored materials are combustible, that could further jeopardize the means of egress and create a hazard to life safety.
In NFPA 101 (2015) Section 22.214.171.124.3 is called Usable Space and states: “Open space within the exit enclosure shall not be used for any purpose that has the potential to interfere with egress.” And in Annex A: “An example of a use with the potential to interfere with egress is storage.” This is also addressed in the NFPA 101 Handbook, where it clarifies that no open space within the exit enclosure is to be used for any purpose that could interfere with the use of the stairs.
NFPA 101 does allow the space under the stairs to be used, but the space must be separated from the stair enclosure by fire-resistance-rated construction that is the same as the exit enclosure, and the entrance to this space can not be from within the stairwell. So imagine enclosing the area under the stairs with fire barriers to separate it from the exit enclosure and adding a door to that space from outside of the stairwell (the space may also need sprinklers) – at some point it becomes more trouble than it’s worth to try to use that area for storage.
Keep in mind that these model code sections are referring to rated exit enclosures in buildings that are required to comply with these codes. Under-stair storage in single-family homes is all the rage right now, and Pinterest is full of ideas. Under the stairs in my house, I’m storing enough camping gear to outfit an expedition of 10 to the Sierra Gordas. And some other stuff. 😀