When most people use under the stair for storage, you can try to have different idea. Use it for powder room.
A laundry room actually needs not large space. You only have to install washing machine and a dryer. The rest, you can easily wash the dirty cloths and left it. A tiny space under stair is enough to be used as laundry room. Even, you don’t need to add door. However, you may add sliding if you wish.
A space at your house that will give your more advantages is under the stairs. Rather than let this area empty, use it as storage seems better idea. You may save any thing as you need. It may be shoes, jackets, books, or even fragile items. Under stairs storage will make your house look tidy and comfy.