Under Stair Garden
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 depending upon the type of space you’ve there. A mini library with bookshelves or a reading book or a home office, if it’s big enough.
The unique and nowadays popular use is indoor garden under stairs. If your space receives some bright indirect light there, or by using artificial lights you can do this. If both are not possible, you can create greenery by fake plants.