The empty space under the stairs can be useful. You can store things there, this is the most common use. However, there are many other under-stair ideas you can opt for, depending upon the type of space you’ve there. A mini-library with bookshelves or a reading nook or a home office, if it’s big enough. A dog house or playhouse for your kids can be an interesting idea too. And, if your kitchen doesn’t offer a lot of cabinet space and it’s near to that under the stair place, make a kitchen pantry.
Maybe you can use this space for an indoor garden. Check out here how to make a beautiful garden under the stairs