To sit comfortably at a table or counter the height of your seat should be approximately 12 inches lower than the table surface. This did not occur to me when I remodeled the kitchen, knocked out a wall, and added a bar / counter that was 48 inches high. For almost 1 year following the kitchen remodel I looked on an off for 34-inch tall stools, they exist, but I was unwilling to pay around $200 each. Then, in early February, I happened upon four 29-inch stools on Craigslist for $100. A few days and about $40 later I had four 34-inch stools.
Before and after, well, the after photo isn't quite done yet. We plan to wrap the bottom of the legs in some sort of shiny black cord to cover the extensions. The leg on the front right of the taller stool is wrapped in sisal (twine).
To make the extensions I first cut the bottom off of a 2x10 hidden flange joist hanger. This gave me two angles which I screwed to a 6-inch length of 2x2. I then slid the extension on the stool and screwed it in place. The last step was determining the compound miter cut required to ensure the stool legs sat flush with the floor. Note that I didn't use a compound miter cut at the old / new leg interface.
Extreme close ups of a bare extended leg and one wrapped in sisal (twine).
As I was unable to find black twine I ended up wrapping the legs in off-white twine then spray painting it black. After many hundreds of feet of wrappings I was very happy to be done.