I'd run (4) 3/4" steel square tubes from the front of the cabinets and 12" past the rear cabinets instead of plywood; it's much more rigid. One on each end and one on each side of the sink cabinet.

If you use a Hercules Universal Sink Harness or a Sink Strap or two, no additional sink support is necessary.

As to the additional price, it's hard for me to say since I don't know if he proposed the job with a built up edge or not.

You use steel supports as Joseph said .

For sure, steel supports. Soapstone is denser and heavier than any other countertop material.

@Joseph Corlett, LLC Thanks for responding! I'm having trouble envisioning what you've said. Where would I secure the tubes to? There is only the thin metal cabinet rail across the front of the cabinets. Would I still need an edge belt added? My initial quote only had an eased edge. If I could avoid the additional edge that would be ideal.

Because this is my first time designing a kitchen. We do plan on installing a cleat on the side wall.

We have a very similar situation. Our soapstone overhang curves from 14" on the sides to 18" in the center. You better believe I drove our contractor, architect, cabinet guy and my husband crazy and gave them all brain damage trying to figure out how to support the stone. Thank you, Mr. Corlett, who has been on these boards a long time and who has put the fear of God in me regarding various design and support issues and always gives valuable advice.

Our island is 93" long with a supporting cabinet at each side leaving 64" of unsupported soapstone in the middle. We used a 2" x 2" x 1/4" (may have been 3/8", DH can't remember) box beam, primed and painted black to blend in with the stone. Our beam runs parallel to the back cabinets. It was placed 9" from the back of the cabinet leaving at most 7" of unsupported overhang in the front middle. This has been in place for 10 years and performs beautifully.

Each end of the box beam rests on 1 1/2" of vertical plywood. The beam was engineered to have no deflection with a 300 lb point load in the center, just in case someone decides to stand on it.

Overall your plan looks good. The only issue you have is that unsupported corner on the right. Your pony wall or what ever that is called falls short of your overhang resulting in a possible weak spot, or a broken corner in your counter top (that unsupported corner). If you could extend that wall to the edge of your counter and beef it up or add a cleat, you might be able to use a box beam the same way we did.

I want to add, your stone is beautiful. And kudos to you for picking soapstone. I hope you enjoy it for many years. Good luck on the project!

ETA: just to clarify, we used a steel box beam, not wood.

I had the same concern with my granite overhang. We put in 3 supports that blend with the back wall of the sink cabinet. They are painted wood.

Mary: The square tube sits on top of the cabinet sides and is covered by the built up edge. You’ll want to beef up the rail they sit on .

Do as Joseph says, plywood is useless as support and in fact I would lose a bit of faith in my installer for even suggesting that as a solution.