Thank you for your input, I went by to ask questions again and I was suggested to just do the install right on top (3CM) and then edge treatment on the bottom lip same as the edge the countertop, a thick overlay. I guess it sounds OK to me. It will have a thicker look to it but I am not happy with the blinds being overly long by 3/4" as it was already made for the existing sill I guess I shouldn't notice it too much.. They do have 1 CM material but I am not sure if the fabricator can order just the small amount to do the job right. You can see the dark line on my window frame was the previous quarter round to hide the caulking, the new sill made of the slab material is only slightly higher.

I don't know if any of you are still around to answer me on this. I too have windows that have gone in and the sill pans didn't get put in beforehand. I also live in low rain Los Angeles. The 40 Fleetwood windows installed even had a custom sill pan included but they didn't get installed. Fleetwood says the lack of sill pan voids the warranty. On top of that my contractor DID NOT prep the sill at all. The bituthene does not wrap over the sill at all, it's just placed around the outside of the window openings. The windows are installed but look really sealed well now with a layers of bituthene over the window flange and he used lots of caulking. He stands by how they are installed and will put it in writing that they will not leak. He says he usually tosses out sill pans! He says his insurance will cover it for 10 years as well. To just add context, I had a garage fire and the property is currently gutted because of smoke damage. 30% of the exterior stucco has been taken off and new waterproof paper will be installed and the browncoat will be replaced then the whole building will be wrapped with a stucco mesh covering to prevent cracking and disguised the stucco damage around the new windows. Apparently the wrap adds some water resistance as well. The roof is being replaced as well. The new electrical is going in so I have time to correct the windows if you guys say I need to do it. It will cost me about $10,000 to reset these windows. Would you do it?

My sink and window set-up are very similar to yours, right down to the window moulding abutting the sides of the cabinets. the only difference is that our sill is much deeper. We have a tall faucet as well. If you're thinking that you need to tile the sill because it will get wet, I can tell you that our sill does not get wet. Don't know why, but it just doesn't. If you want to, you could trim or remove the part of the old sill that sticks out horizontally and use a different piece of painted wood trim applied on its side. By this I mean so its front is flush with the front edge of the moulding on the side of the windows. This is what we did. We have a piece of moulding about 2 inches high and maybe 5/8 inches thick. The side window moulding runs into it. So it's like a frame to the window, but a different profile from the side moulding. It looks like it belongs there. We needed a flat moulding to have room for our faucet. We tiled up to the bottom of the cabinets, and the tile runs into (and stops at) the moulding on the sides and bottom of the window. It looks nice. Sorry I haven't learned how to post pics.

Sills and extensions jambs from the local lumberyard and or site milled were it me.............