HI -- 42 inches is a standard bar height . 15 inch over hang is good for knee space .

Thanks so much!

Build the wall 40 1/2" tall and when you place the counter on it, it will be the standard 42" bar height that 30" barstools are designed to work with. Be sure to build in the support needed for any overhang. If you use stone, you will need that support to both keep them from tipping, and cracking when people lean on them.

Thank you for the information. We greatly appreciate it!