I have a new garage addition that is going to tie in perpendicular to the existing house. I swiped the following photo from another question on this site; my apologies to the original poster. In the picture, imagine that a garage would protrude forward from the right side of the picture. The left side of the gable roof will have to tie- in where the two different heights are on the current structure. I am a little puzzled as to how to handle the two different roof heights when I tie in the new gable. Any thoughts?

I would guess that you will have to tie in the new roof... level along the side of the house... and address the space that is exposed by removing (if that's what's happening) the existing roof line. You'll probably end up with a triangular area that will have to be wrapped, insulated and stuccoed. Don't neglect the flashing that you'll need to channel water down the slope necessary to prevent leaks where the new roof meets the existing structure. Good Luck!