Mawari Kaidan (廻り階段) – a U-turn staircase. Although there are many types available, with various pros and cons, there are some commonly used industry standards. A 13 or 14-step staircase between 2.8 meters of floor-to-floor distance are the generally accepted standards for residential homes.
As far as how many tapered treaders to allot to the staircase, there are a couple things to consider. The more tapered treaders the more awkward to use, due to there being less area per treader for your feet to contact. Remember, you only have 180 degrees, and it’s not practical to have to circle wide as you climb the U-turn for lack of space nearer the vertex.
For this reason, 4 (四段廻り; yondan-mawari), 5 (五段廻り; godan-mawari), or in rarer cases, 6 tapered treaders are the standard for residential staircases. Note that a 14-step staircase will generally have to use the 4-step (or 6-step) U-turn, and the 13-step the 5-step. This keeps the straight portions of the staircase below and above the U-turn congruent in both angle and length.
It is of course possible to stagger the start and end positions of the staircase, thereby allowing for an odd number of tapered treaders (and this might be what you’re in fact trying to achieve). This just means you have to compensate for the loss or addition of a tapered treader by the same loss or addition of straight treader below or above the U-turn. Naturally this results in a staggered start and end position.
Japanese law requires a minimum staircase width of 750 millimeters, a minimum tread board width of 150 millimeters, and a maximum step height of 230 millimeters. Tapered tread boards must have a minimum width of 150 millimeters between 2 imaginary 300 millimeter legs starting from its vertex. (By this code, basic trigonometry — the law of cosines — will tell us that an evenly sized 6-step U-turn is the upper limit.)
Adjustment within these parameters set by law can be done by altering staircase angle, number and height of steps, and number of tapered steps. One thing to keep in mind is that if you’re thinking of utilizing the area under the staircase for anything such as a toilet or closet, you’d probably want to have as much usable space as possible.
In this case you’d want to maximize the angle and step height, as well as possibly increase the number of steps in the first half of the staircase and U-turn section. As an alternative you can drop the floor level under the staircase to accomplish the same objective if the staircase angle and treader situation can’t accommodate this design.