Probably a bit late in the day now, but I made one of these chair/steps combinations, which I posted here:
http://lumberjocks.com/projects/47287
I haven't read all the posts on this topic, so it may already have been mentioned that there are a couple of downloadable versions in the Sketchup 3D Warehouse - search for Library Chair/Stool.
You might also take note of my comments about using 'bar flap' hinges. They are an improvement on ordinary hinges as there are no pivots projecting above the top of the seat panel. However, they do require a lot of work to get the recesses for the third part of the hinge cut into the seat edges, especially as these edges are not at 90 degrees.