*I'm sorry about the lengthy description. I'm photographically challenged at the moment.
I went about installing a two side by side, double hung Marvin integrity windows. After doing that, my dad said, "wouldn't it be nice to have a large window sill on the inside?". I went to look at some recent construction. The carpenter had put in window sills on all the windows. At a glance I thought that he removed the entire bottom jam of the premade windows, replacing it with a larger piece of wood. I can't do that now.
These Marvin integrities have a strip of wood about 3/4 of an inch high which conceals the bottom of the window sash. My best guess right now is that I pull these off and put the window sill on that same plane, right up to where they butted to. I pulled one off and noticed that there was a vinyl barrier in front of it, and a foam strip inbetween them that I sort of ripped. So, maybe it would need a silicone bead there.
I want to clarify with you that this is the best way to do it. The other thing is whether I cut the vertical jam out to a bit to slip the sill into, which would look better than profiling the sill to fit it. I'd like the sill to be 1 inch thick, but that would make the bottom of the sash look shorter. I feel like its not quite proper, like maybe I missed the opportunity when I installed it. The alternative would be to stick it on the front, level with the jam, but then you would a seam.
These windows have the same strip of wood concealing the bottom of the sash. If I remove it, there is a nice flat surface.