Does electrical outlet in man made quartz backsplash require box extender by code?
I don't know what all the issues are that influence the code, but here is what it says:
2017 NEC 314.20 Flush-Mounted Installations. Installations within or behind a surface of concrete, tile, gypsum, plaster, or other noncombustible material, including boxes employing a flush-type cover or faceplate, shall be made so that the front edge of the box, plaster ring, extension ring, or listed extender will not be set back of the finished surface more than 6 mm (1∕4 in.).
If your electrician is trying to fit an Arlington BE-1 which often does not fit well maybe he could try a Raco 976.