Space and light are important factors
Yes, the first thing that you should consider when setting a pebble garden under the stairs is the space available. Choose as many plants as can fit in there and consider them to be low growing, so that they won’t hit in the wall. Light is also really important for growing plants. Most often the space under the stairs cannot get an amount of natural light, because in most of the cases there is no space for a window. And in such cases you should add a grow light.