Mixing 2×12 with i-joists on 1st floor

My wife and I are working on building our house.  There will be a basement with one floor on top of that.  In the main (larger) part of the house, we'll be using 2×12's for our floor joists.  They'll go from the outer walls to a middle support wall.  On the narrower part of the house, we'd like to use i-joists to span just under 20 feet. 

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I realize that the depth of 2×12's vary and are generally smaller than 11 7/8' i-joists.  But knowing this, how is one to use traditional lumber and i-joists and still have a sub-floor that is flat/level?

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Ideally, I do not want a portion of our sub-floor to be 1/2' higher than the rest. '

I've attached a picture that I hope illustrates what I'm talking about.

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