Steel-on-the-flat isn't good countertop support. I'd have the bottom of the peninsula top rodded with stainless steel and epoxy every 16". Plenty strong enough for a 14" cantilever and no leveling issues.

"What makes the difference on if someone needs supports every 16” or 24” or something different?"

ocean78 asked for a method of supporting the top over full overlay doors. I'd rather insert 1" square tube into the cabinets, but there isn't room.

"Will this option impact the integrity of the stone at all?"

Yes, it will make the stone about 650% stronger.

My cabinets are considered full overlay, but my counter fabricator had me specify a small gap at the top of the island doors to accommodate steel brackets, spaced 24” apart (along with solid cabinet side walls) It’s a 110” x 55” island with a 15” overhang for barstools

Yes, I definitely interviewed fabricators with an approximate cabinet plan that had not yet been finalized/ordered. I wanted their feedback so I would get exactly the features I wanted accommodated. One fabricator said you don’t need any support for your overhang. Seriously, what would a 110x55 granite slab weigh? That fabricator cut from my list. I remember my ultimate choice said, I wish more customers came to me with a cabinet plan before it was ordered.

I could dig through my files later today, as I can’t remember the brand name of the bracket system, I just remember I paid for a thicker heavy duty bracket as opposed to a base line bracket. I was too nervous to watch the installers at work (!) but I do think it was just the brackets. Found a picture of the island before granite - I think the brackets are installed into small channels by the fabricator, because I don’t see any signs of channels here.

Sorry, but steel-on-the-flat is nearly useless to support stone. I don't care how thick it is.

How do I know this? I have to remove tops regularly. To remove tops, you drive shims around the perimeter and between cantilevered flat steel and the bottom of the stone. Every single time the steel flexes.

Joe, can you clarify what you mean by steel on the flat, are you referring to what the OP's kitchen people suggested, a flat sheet of metal? Or brackets like I have? Got me a little concerned now

Joe - as I think OneRidge is, I am trying trying to understand what exactly you mean. My confusion comes from the fact that most of what I see to buy for support is like the below. But this looks to me like steel “on the flat”. Although maybe I am thinking of it wrong or not looking at the picture right. Can you give a link or picture showing what you would recommend installing to support a natural stone overhang?

19th post down, please.

The above-pictured brackets are technically steel-on-the-flat, but they extend all the way to the back of the cabinet fronts and they're a half-inch thick. With that spacing, they'll be fine.

There are ways of doing it. From a top buildup, to pilasters in between each cabinet, to a pony wall insert and ordering shorter doors, to s complete custom design.

The solution needs to be custom designed to the individual project requirements.

MIA standards are 10” and not one skosh more without support.

Skip the cabinets on the back. Add backing, stretcher bars, steel, and posts. You then can have room for seating on 3 sides instead of just 1.

Hello, what was the final decision for this we are in the same sutiation now

Hi Emily - We had the 12” overhang installed with the cabinets as is. We called around to several fabricators in our area and about 50% said no more than a 10” overhang should be installed and the other 50% said 12” was their standard overhang without supports. We even had someone else, who installs supports for counters, come take a look and he didn’t think we needed them. Not sure if it makes a difference, but our peninsula is 37.5” wide without the counters. We don’t place any heavy objects down on the overhang and usually put our groceries on the island when we bring them in, which we would have done anyway so not a big deal. Definitely a lesson learned for us if we ever do another kitchen remodel (which won’t be for a very long time!). Good luck with your project!