Many things can be done to either decorate or make the space under a staircase usable. The angle of the space discourages some people, but with proper planning and some creativity, it can come alive and become as useful as any other space in the home.
Turn the area under the stairs into storage space. Add a simple book shelf or wicker baskets for storage, or purchase shelving that is designed for use under stairs.
Relax under the stairs. Place a comfortable chair or chaise longue, your favourite potted plant, and hang a few pictures in the space. Use it as a reading spot, or as a place to sit while you take off your shoes when you walk through the door.
Put a half bath under the stairs. This is a major remodel and will cost the most, but it could add to your home's value. The highest point under the stairs should be eight to 10 feet to create a bathroom. You must also have space for a small pedestal sink. If the stairs are next to another bathroom or the kitchen, it will be easier to run the plumbing into the space.
Study under the stairs. Place a computer desk and desk chair in the empty space. If you will place a computer in the space, you'll need to add a power outlet. Don't forget to add a desk light if the area is not well lit.
Put your kids under the stairs. Not literally, but allow your kids to use the space as a toy storage. Since many kids' bedrooms are upstairs, toys get dragged downstairs and left in the living and family rooms. Set up plastic bins and cubbies for your children to store their toys when they are done playing.