Bedroom under the stairs
When designing that space under the stairs you have no idea what to do with, you don't have to go too far. If the space is not big, put a big storage box, a mattress on it, add a pillow or two and that's it. If you want to add a bit of touch, shelves with your favorite things, make some space for books you love to read before sleep and you just got yourself a nice, little, personal cave. In small spaces use bright colors to visually expand the space and don't go in extreme with details.
If you have a little bit more space, you can make very elaborate tranquil place for sitting and reading, or even sleep like in a "normal" bedroom. Wall can be used for shelves for books, things and other stuff you'd put in your bedroom plus some details that can be found in a living room. You'll have to have some custom-built furniture but this is the only way to use every square inch of that precious space. Add to that a night lamp or even a small TV and we bet you'll have hard time leaving your new piece of paradise.
However, you can go even further. There are many houses with plenty of space under the stairs, so plenty that you can literally put a normal-size bed under it and have another space for yourself or a guest where nobody will feel constrained. This is great way to go if you have an open concept space because you can connect your "bedroom" with other parts of the home and make something unusual. Just because everyone has a bed behind the closed door that doesn't mean you must have it too, does it? So, go bold, go brave and you'll have something that really stands out.
If your apartment was designed in a minimalist style, or something that resemble Far East, you can use the space under the stairs, too. In that case, one simple bed, not too much elaborated or not elaborated at all for that matter, will do. A detail or two will make the space more warm and you can choose bed linen to suit the rest of the room design. And that's basically it: you will get a useful space, a nice focal point in your apartment, and all that without too much effort. This is an ideal idea for smaller spaces where every square foot counts. ■