Couple of questions regarding the design of the stairs.........
1. How high is the mid-landing in the design? and
2. How is it constructed?
For any stair there is quite a bit of carpentry involved that will never be spelled out in plans. For a good overview see page 164 of Wagner's "House Framing". It covers a straight stair. In a u-shaped stair you locate and build the landing at a height midway between the two finish floors. You then build straight stairs both up and down from there.
The second of the books I suggested (http://www.countryplans.com/books.html) to new builders is Nash's "Do It Yourself Housebuildng". See chapter 20, p. 515 for a detailed discussion of stair framing of all types.
You will want to make a bit of a study of this for your first stair... and having an experienced carpenter's help to call on is a good idea as each stair will be a bit unique.
I've been trying to recall where I saw information on this. Wouldn't you know it, it was in my copy of Wagners House Framing. just like you said....page 164. Guess that's what happens when I have too much on my mind.
Thanks !!!