one -- We ended up with a deep sill at our sink window, not because we bumped out the wall but because the wall had to be furred out, four inches into the kitchen. Our sill is about 8" deep. There's a 3" soapstone backsplash and about 3" of painted apron below the sill. We considered using soapstone for the sill, but didn't because it would have looked so much heavier/darker. Also, the fabricator balked at making the narrow little "ear" piece of the sill shape, that extends out to each side, without breaking the stone. Given the water concerns (from plants and splashing from the sink) we chose to make the sill and apron out of vinyl Azek boards. Once painted, it looks just like the wood on the rest of the window trim. By the way, I wouldn't recommend putting double hung windows over a counter. It is hard to reach them to open and close. and two -- Here's what the boxed-out windows look like in the living room and dining room of our 1920's bungalow. The picture is of the living room, but the dining room is the same, though only three windows wide. The kitchen window trim was modeled after these. The ten inches you have sounds like more of a perch than a seat, I think. Ours are comfortable, at 15" deep. (This is, far and away, EVERYONE'S favorite spot in the house.) They have no back, though, and they're 26" off the floor, so they're not really suitable for dining. If you were to lower the seat and add a comfortable seat back below the window panes, it could work very well on one side of a dining table, especially if you built the bench part into the room enough inches to give you a comfortable seat depth.

This is quite after-the-fact, but I can tell you that STC varies enormously by product type. If Pella is saying 27-35, then 35 would probably be for something heavy like an outswing single door with midrail and laminated glass, and 27 would be for something relatively leaky like a horizontal sliding window. Construction quality makes a huge difference as well. A luxury manufactured window like a Loewen casement starts an STC 33 with standard IG, and can get up to STC 40 with Loewen's acoustic glazing offerings. Of course, Loewen is easily double the price of Pella. Still, truly "soundproof" starts around STC 50, and only products built specifically for acoustic attenuation can meet that criteria. Also, your typical 2x4 wall only has an STC of about 33, so once your windows match the STC of your walls/roof, windows cease to be the primary source of sound infiltration.

I would also look into different ceiling installation options because 118" will need at least a couple of middle supports if you installed on the sides.... 96" is a very manageable ht to diy! I have regular sized sliders (that are recessed about the same amount as yours except with a lot of molding around them) that I hung a tension like curtain rod on except it is mounted on both sides... I hung open weave linen sheers on them and almost always keep them pulled to cover just one half... anyway, I couldn't have used the center supports with this rod even if I wanted to because I have the rod hung tight near the top like you need to do-- in my case it works because I only have sheers hanging, so less wt and i don't close them too often, so very little tugging on the rod...

Can we see the whole space ? IMO shutters block light and view I think it is nice for a child to be able to see the trees I would do a roman shde in a fun fabric and honestly it does not even need to function since you have a blind.As for the seat you do realize that that window glass should be safety glass . I mention it becuase my cousin put his head right through a window above a window seat and needed many stiches.