Our NYC apartment building has very deep exterior ledges, extending around a foot beyond the window, but 2-3" below the window frame, so a window AC can't rest flat on it. How do we appropriately brace/mount the AC? none of the standard mounting brackets will work, as they don't extend far enough out to reach over the ledge. We don't want to just prop it up on blocks, as everything I've read suggests this is illegal. Any advice appreciated!
I strongly recommend asking your landlord. They will have dealt with this before, and will know what solutions will work -- and given the potential liability issues they may insist on doing the instillation.
(I have dismounted a NYC airconditioner, which had been installed with a moderately complex metal framework apparently designed to solve exactly the problem you are describing. I don't remember any details, though.)
I have the same problem, plus the inner bottom lip of the window is several inches higher than the outside sill, so that the little adjustment blocks included with standard brackets don't add up to enough height.
Is seems from the city's website that you don't explicitly need to use one of these brackets to install your AC. As long as it is secure you should be ok to use a different kind of structure.
I have been thinking of having a sturdy piece of wood like a 2x4 cut to the width of the bottom window frame (so the machine's weight doesn't rest on the thin lip). The ends could be shaped so that they stick in the sash rails and it can't be pulled out without raising it. Then have additional pieces of wood or metal angles made and connected so they stick out and down, creating a shelf for the AC and a 'leg' that would rest on the stone sill. The sill projects as far as the AC itself, so there is no problem finding a solid support surface. Clearly, everything needs to stay together securely, and it needs to be fastened to the building itself, so I may drive a couple of long screws down the long wood piece into the bottom window frame.
It seems this is completely legal as long as everything is securely fastened to the building.
Also reference this article which I found very useful: https://www.brownstoner.com/forums/topic/window-ac-support-bracket-install-with-concrete-sill/