Your marble "stool" looks nice. Is this a kitchen window? Over the sink? Will there be a backsplash that comes up under this? Google pics of Craftsman window trim. Remember if you only use 1 x's, you should round over or chamfer the edges. A square edge will not hold paint well.

What is the trim style in the rest of your house?

Hi, this room we call the “spare” room, it’s kind of like a utility room for maybe TV and gym equipment, no tiles or backsplash involved.

We plan on replacing all trim - this just happens to be the very first window replacement.

I like the style of craftsman trim style but I wasn’t sure if trimming out without the overhang in left and right side of each side casing would look bad or unfinished. Or if there was another style I should go for instead.

Depending on what else is going on in the room (or the rest of the house), I think you could make the apron under the stool extend out for the casing sides to set on top of - if it's only this one window. If there are several areas where you are modifying, then maybe change to a more traditional or transitional style. You need to have a plan that is cohesive all the way through.

Any possiblity of redoing this stool or "adding on" the needed extensions in marble?

When it comes to trim (or other details!!), you have to start with the total finished look to be able to get there! Ask me how I know!

This is the style I’m leaning towards, in my case the sill/stool will look like it’s cut short, or not wide enough.. maybe I’m just overthinking it.

So these two red highlighted overhangs wouldn’t be there. I was almost thinking of “extending” the marble with wood.. not sure yet. Since the side casing is same flat and thickness as apron, maybe I would get away with it ? I prefer to start the first window in this fixer upper the correct way, maybe it could be a new design and catch on ?

I thought of a way around this, in this photo, regular wood casing, stool, apron etc. Then just the marble on top of that wood stool. Only downside I can think of is the reveal around the window won’t be consistent.

Not too bad. What is this window used for?

It’s a 48 x 48, my actual window is in the very first two photos. It’s egress type, in case we decide to turn this room into a bedroom, but right now it’s just a general / all purpose living room.

I would go with drywall returns or a simple wood stop with that sill ir just do a wood with horns like the last pucture . Typically I like all the rooms to match.

The marble will stick out beyond the side trim, so now I can’t do the last picture idea (marble on top of wood). I’m leaning towards wooden stops, kind of end caps each side, and hoping it won’t be too noticeable.

I looked for correct size marble and can’t find anywhere, the big box stores have I think it’s a man made type marble or something, scratches way too easily, all their pieces are damaged.

just modify the last picture. it's fine if the marble is proud. do you trim casing around it.

I did something similar w/teak on my own window. I wanted a plant shelf. this was done awhile ago, so the white trim casing isn't what I would use now. but this should give you an idea of how to work around your marble. (if you have any left, cut the marble for the inside trim. I did it w/the wood on mine)