I want to know if a countertop microwave can be placed on a shelf under the counter. If so what is the code required clearance for floor to bottom of microwave distance? I live in California.
The issue is not code. The issue is manufacturer's instructions. The good news is that you can find the manual for almost any current or recent appliance online, either through the manufacturer's web site or often through Home Depot or other retailers. Find a model or 2 that you like based on size, features, etc. and then download the manual to find out clearance requirements. Very often the requirements are fairly low, which is due to the way microwave ovens (as opposed to conventional ovens) work. Floor to bottom of microwave is not likely an issue at all - except that you want it at a height that is comfortable to use when removing hot items. The issues are normally back and/or side and/or top clearance for proper airflow.