I'm bumping this, would love some feedback before I talk to them later today. Thanks!

No.

Why only a 12" overhang? That's not very much. It seems like you want to go the cheapest route not matter what here.

Maximum unsupported depth is 10-12” for most materials, regardless of % of overhang vs cabinet. You can get hidden supports for the additional depth, which is what I recommend. You will be much more comfortable sitting at countertop. Enough space for knees.

You don’t want the legs at the corners to be the only support for weight of the countertop. An option would be a balance running from one leg to the other.

The island needs to be firmly attached to the floor. Else you run the very real danger of the whole thing falling over.

You’re not putting expensive stone on top of that???? Forget the overhang. Fire the cabinet maker. That scrap wood and pocket screws is not going to work. There’s no shear strength in screws joined that way.

Mark_rachel, its only 12" because there is door to my backyard to the right of it and a wall that juts out behind it that would create a bottleneck if the island is too big. That's the most I can have without blocking traffic. When it was designed I wanted a full two feet behind the sink so my kids could sit there without their stuff falling in the sink. It was decided to put cabinets in the back for a little extra storage without the need for extra support. I would love to have a slightly bigger overhang but I don't have to room and yes, I want the best I can get for my money. If you could have the exact same countertop for $2200 vs. $4500 isn't that the way you would go? I'm bucking at the supports because that is what the back cabinets were put there for, supports are going to get in the way of the doors.

Mainenell, that's an interesting idea! It would require adapting the back cabinets though or it would block the doors opening.

Weedmeister, thanks for the warning. It is firmly bolted in. My husband built the base on site because this house is old and the floor decidedly unlevel.

"Fire the cabinet maker. That scrap wood and pocket screws is not going to work. There’s no shear strength in screws joined that way."

Rarely do I disagree with lwo, but she is misguided here. Those are not drywall screws with little strength in shear. They are very strong screws designed for that purpose.

While apparently not as strong as regular joinery, the pocket screws are completely adequate and appropriate in this application. Plus, you saved the cabinetmaker lots of time which saves you lots of money.

I'm a bit biased, because sometimes I can avoid tapping a cabinet front off to change a sink by simply removing 4 screws and a style in a sink base cabinet.

"Given this configuration and 3cm quartz am I right to assume any additional support is somewhat redundant?"

Yes. Engineered stone is 35% plastic, giving it much more strength than natural stone. Cantilever to 15" unsupported as the manufacturer allows.

Thank you for the assurance Joseph, on both the safety of the overhang and the joinery! HD would not budge on their policy, which I expected. Since they are such a large store they take the safest route liability wise. I spoke with the fabricator who agreed that it wasn't really necessary and the quartz manufacturer would still warranty if it were to break with only a 12" overhang but he couldn't install it without them. We went ahead and attached 13" flat brackets that come out an additional 6". To me, they frankly look like they'd be completely useless if they were actually needed, but since they are not I will not be concerned about it. They are just to meet the HD requirements. The countertop goes in tomorrow and I'm very excited to see it!

can you share the link to LG Viatera website where you found the overhang guidelines?

Sorry, HU-459777929, this was about 3 years ago and I no longer have the link. I checked out the LG Hausys (Viatera) website but couldn't find it. It's changed quite a bit. I did find a Viatera Fabrication Guide with those specs on SlideSide with a google search: https://www.slideshare.net/MarbleConcepts/viatera-fabrication-guide

Here's a screenshot of the overhang guidelines:

By the way, I got such heck for the frame of this island, but it turned out beautiful and so did the countertops! These were taken along the way. I've never done a finished shot of the kitchen. It took too long!