Integrating storage into stairs like this makes so much sense in small spaces. It is common in Japan and I have seen Chinese furniture like this, but Voet-Theuns architecten have given it a modern spin.
A commenter at Materialicious notes that "in Belgium (the country of Voet-Theuns, and probably also the location this project) it’s quite common to have a ‘loft bed’ in student rooms. That’s because these rooms are quite often in old houses, with tall rooms. The landlords make into the room a second ‘floor’ for a bed (the height is only 1,5 meters, but that’s all you need to sleep), which saves a lot of space in the room."