Our contractor did it. It had to be ready when the granite was delivered. Our cabinet maker also would have done it if we had wanted corbels.

Contractor and it should be on the kitchen plans and scope sheet.

The cabinets installer did mine after discussion with the granite contractor

The granite guy approve the rods for the overhang.

No self-respecting stone fabricator will install on supports he deems inadequate. This should be coordinated between all the players. Speaking as a repairperson, be aware that your fabricator's competitiveness is not working in your favor in this instance. Let him know you've got some contingency money available to do it right.

Our fabricator installed steel plates for our marble overhang when he arrived in the morning to install. He notched the cabinets, installed the plates, then installed the marble.