You may have figured this out by now from the contracter, but countertop people should do plywood underlayer. It has to be to guarantee the right fit to the quartz - too many things can go wrong if someone else does it. They screw the plywood into the cabinet. They use a epoxy-like cement between the quartz and plywood - they will float the slab on a large amount of gooey glue, then while the glue starts to cure, they adjust the height with wood screws from below to level precisely. Then the glue sets within 10-20 minutes or so, then it's done.

First, I would question the professionalism of your installer. Underlayment is not the same as industrial grade PB, that being said, MDF or plywood would be a much better choice, and yes, I would use 5/8" material rather than the 1/2". Especially if you have Euro cabinets you will run into problems. Spanning the DW area should not be a problem, just make sure they don't put a joint close to that area. FYI Particle board comes in a number of different densities. This indicates how much wood and how much filler or adhesive is in the sheet. Industrial board is a 45 lb density, commerical board is 42 or less while some underlayment is less than 20 lb density. MDF is typically over 50lb density, and very stable, some are even exterior grade and water resistant. A contractor that buys based on price at a box store, is, in my opinion, not a professional. Good Luck !

boone, I don't know if the same practices are used in SoCal as NorCal. I don't even know if the thickness of the plywood substrate is spelled out in some code. Does that 1/8" really make that much difference? Apparently my fabricator thought so. And my GC spelled out 5/8" specifically in our contract which was not otherwise rich in detail. Typical of him, I think he "forgot" and thought he could get away with reusing the old plywood from 1991. It does seem to be the norm in these parts (North Bay Area) to use 2 cm slabs on plywood substrate and a built-up edge. I only know and understand this after the fact. Please check with your folks down there, because I don't mean that my experience will be yours. It was a minor big deal for me, fabricators standing around fuming about the wrong thickness plywood, saying it would never do.

I am a fabricator. A plywood underlayment for 2CM is typically complemented by a 4CM drop edge which masks the underlayment on the edge. the plywood is really just a spacer as it does not contribute significantly to the structural integrity of the overhang. For 3CM quartz a "fussy" fab shop will grind the underside of the overhang to remove the printed Logo's from the OEM. If requested the fabricator can polish the underside but the fab shop polish will not be of the same quality as the surface. However, it will be good enough to look finished on the underside. Polishing the underside is NOT an option typically offered by the fabricator. In my business we've learned to ask the question when the layout of the room, and particularly the stairway location, means the underside will be readily visible on a regular basis and not just when someone is on the floor looking up. Another option is to make the overhang "double thick" where 2 pieces of the engineered stone are mated back to back so a factory finish is on both sides. This is an easy option for 4CM assuming corbels are used or the double thickness is extended back over the cabinet. Finally, some natural stones that are crack prone have a fiberglass matting epoxied on to the back of the stone to help it hold together during transport, fabrication, and install. A competent fab shop will remove this backing as a matter of course. I am always annoyed at the poor quality evidenced by a job where the backing can be felt and seen on an island overhang and even on the narrow countertop overhangs. If you really want a great looking underside the best way is to double up the material back to back. It's an expensive way to do it for a variety of reasons.