It’s probably legal to store combustible material under an exit access stairway (i.e., an exit stair that is permitted to be unenclosed) even though it wouldn’t be allowed under an unenclosed exterior stair or an enclosed interior exit stair. But it’s probably not a good idea. I tested the combustibility of the foamed plastic used as display stands for architecture (and other) reviews and exhibits, here shown stored under the exit access stair in the domed Crit Room in Milstein Hall, Cornell’s architecture building designed by OMA. This video documents my combustibility test and explains the code issues.