You don't want to read the comments on my post asking Why are Japanese houses so weird? Perhaps it was a poorly phrased title. This house doesn't fit in the weird category at all, and has some very nice features. Architect Yuki Miyamoto has made 750 square feet feel huge, by going tall.
The concept of the design is to maximize the use of existing natural resources like, Sunlight, Wind and Greenery, to create a healthy residential environment and not depending too much on technology. To make use of the natural ventilation, high ceiling is more appropriate for the hot and humid climate of Tokyo. And the windows set between 2 treehouses fill the living room with soft natural lighting.
This is classic green environmental design, keeping cool with high ceilings and natural ventilation; scoping out the views and placing the windows carefully; designing the house to catch the wind.
The typical plan was bent like a dogleg, which was designed to face the big old cherry blossom and enfold the rich greenery and intake cool wind from the west. In addition, although the area on the 2nd floor is quite small, 35sm, the dogleg shape can give more expanse and depth to the space visually than a rectangular plan. The high ceiling and the existence of the floating bridge can also add a vertical expanse.
Note also the design of the bathroom on the ground floor; even in a house this small, it has a proper datsuiba, or changing area with the sink, a separated bath so that one can take their time soaking in the tub while the other uses the sink, and of course the toilet is in a room of its own. See more on this in History and Design of the Bathroom Part 6: Learning from the Japanese
It's the stair of the week too, with cantilevered treads and yes, it has a handrail.
It's amazing how big 375 square feet looks when all it has in it is a little kitchen and the rest of the space is open and uncluttered. More at Yuki Miyamoto's website.