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They are installing today ... can they install supports after the fact?"

Yes, ugly knee-knocking corbels, but structural steel no.

Take a picture of the plywood please. I'll bet it's wavy CDX instead of cabinet grade. I'm not a fan.

Hope these are good!

So you're going to be looking at plywood on your waterfall end?

Stone fabricators don’t alter cabinets or design counter supports. They just install stone over what’s there, if it’s appropriate. That design and install of an appropriate counter support is the responsibility of your General Contractor and Kitchen Designer.

Yours failed in their responsibility and the stone install should be canceled. That isn’t a workable anything design. You have to go back to the drawing board with GC and KD.

Ive read that the subtop can be sufficient, but there are risks. Anything I can do moving forward?

Lock your GC and KD into a room until they come up with a real actually thought out engineered solution that doesn’t have raw plywood hanging out in public shaming them.

"Ive read that the subtop can be sufficient, but there are risks."

Did you read that for your exact application? Are you will to take the risk of it failing and falling on a child or parent (anyone for that matter)? In other words you read wrong. Follow LWO advice above.

They've put the marble right on top of the plywood. No brackets or supports. I did tell the contractor we would want supports but this is how it was done... Will see what he can do now.

The only thing that can be done is the stone removed , and that entire island be redesigned correctly. With cabinets and doors on the rear, there’s no attachment point. With the ugly plywood waterfall, there’s no concealing that.

You should have cancelled the stone install. That would have prevented the much bigger mess that you are in the muddle of right now. It’s an injury lawsuit waiting to happen, and you are fully aware of the hazard, which makes you culpable. You need to bless out your GC and KD, and force them to resolve this. It was their job. They failed in their job.

What is the plywood waterfall you mention? Here's a few other images. You can see the finished cabinets underneath. The wood is the finished cabinets and under the countertop is the white subtop.

I trusted their professional opinion and regard that this slab does not need any more support. But if I wanted to put more, could I do so with the wall? It wont be every 24 inches but at least something. The only thing holding up the marble waterfall now is the subtop. The GC and fabricator thinks it is fine. I might be more risk averse but it sounds like the community thinks they are wrong and there is too much risk.

Finished cabinets underneath

The white bottom of the subtop can be seen under the marble countertop

Let your GC argue with the industry standards. Sub tops are a thickness compensator, not for stone support. Plywood flexes. Stone does not. It breaks.

https://www.naturalstoneinstitute.org/default/assets/File/consumers/homeownersconsumer_countertop.pdf

"The GC and fabricator thinks it is fine. I might be more risk averse but it sounds like the community thinks they are wrong and there is too much risk."

They are and it won't be their ass that is on the line when it falls on someone.

Just so I understand correctly, one of the main risks is that a part of the large slab of marble could crack, and fall if not fully supported? Either the waterfall and countertop, or just the countertop?

I want to demonstrate the issue well to the GC and fabricator so they will fix it. Thanks all!

Print out the booklet that discusses industry standards. Highlight the issues that do not comply. Including exposed plywood in the waterfall. Show them pictures of how it does not comply. Ask for their signature next to those sections, acknowledging that it does not comply. . Ask them to remedy their improper design. In writing. Everything in writing.

News is I have shared this with the GC and he said since it is one large slap of marble on top, it will not break. I am trying to get him to add supports but it looks like I will need to go with another GC.

"I might be more risk averse but it sounds like the community thinks they are wrong and there is too much risk."

Just so you know, the folks offering you their opinions here are true, respected contractors and experts. You can click on their names and see probably thousands of posts offering their expert advice.

Personally, I would take Cook's Kitchen's advice and present your current GC with the written industry standards. I am sure your GC is aware that installing dangerous and expensive items like this, over a reasonable customer's objection and presentation of code requirements, could get him/her reported to the licensing agency, stuck in court, and suffer the consequences that some truthful comments on Yelp and other social media would bring. You should force the GC to re-install, not get a new GC. My goodness, with a new GC, you may have to sign away your expectations that the slab could be broken when uninstalled and reinstalled. I would leave that as a LAST resort.

https://www.houzz.com/discussions/5802914/help-countertop-sitting-area-overhang-12-or-15-inch

I thought this sounded familiar. The OP reported in October that the GC had agreed to use supports. What happened?

He just didnt do it, he had said no problem, but it wasnt done. it's a shame. Yes that og post was so helpful in making a decision and the GC stuck to that size. Still talking so hope we can add brackets or replace. He is somewhat receptive.

"and he said since it is one large slap of marble on top, it will not break."

Oh it will be a large slap alright. He. Is. Wrong. Period. Stop.

Someone kept paying them instead of doing a Stop Work and Stop Payment until things were fixed. That fix required redesigning the island, which is why the GC ignored you and kept going. He thinks it’s easier to ask forgiveness than to tell you he is clueless and “it’s already done, we can’t do that now”.

The only way you’re getting this guy’s attention is if you cut off the money and throw the workers off of the job site. If this is a tract build that the builder owns, and you’re only buying at closing, , that isn’t possible. That’s only possible if you own the property , and are controlling the build.

If this isn’t your house yet, have to decide if owning a house from someone who deliberately does things outside industry standards, that he agreed to correct, and then went back in, is worth it to you. There’s probably a lot more questionable things done that you just can’t see. I’d be talking to a construction attorney about getting out of that contract.