A king-size loft bed is certainly an unconventional piece of furniture, but it saves more than 40 square feet of floor space. This project will take a few hours. These plans are for pine and plywood, the cheapest woods available, but you can substitute more expensive lumber for a fancier finished product.
Here's another King-size loft bed. Looks like they use this room as an everything room, for a studio apt. perhaps. I see bed, desk (under bed), and sofa. This could also be just the bedroom if you don't have room for an office elsewhere chez vous.