A picture please and usually a window seat is best at least as deep as a chair seat and if you want to put your feet up and recline I would say a bit deeper . Post the pic here in a comment DO NOT start another post

a built in bookcase on one side, what is on the other side? How does it relate to the other furniture placement in the room? What is the size of the window? Yes, photo's of the potential location would help.

Sit on the floor in the corner of the room with your legs stretched out along one wall, sitting far enough away from that wall to comfortably read a book. Measure from your belly button to that wall, double that measurement, and that is the minimum width (from front to back) of the window seat.

Detailing the window seat to fit into the space is a job for a competent local designer or local architect.

This is the proposed window seat, however, it seems rather short in the depth, but is long enough I believe.

It should be the same depth as the cabinets. Twenty-one inches deep is plenty of room; most dining chairs, for example, are 18" to 20" deep.

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If you want to lounge on the seat, I would think you would need at least 21 - 24" in depth considering pillows and elbow room. I think I would be more inclined to do a great overstuffed chair to melt into with a good book, an ottoman and little side table .

Good luck!

Using Mark's suggestion, the comfortable depth for me to sit sideways stretched out on a window seat would be two feet so that I won't bump my elbow against the window while holding a book. YMMV.

Great suggestions, thanks all, and great pictures Sabrina!!

I'll guess it's drawn at 16". I'd do this.

Are you sure this will be the location of choice to read a book? Things like this do not always turn out as planned. I had a friend that built a bluebird house and secured it to an old dead tree trunk. The bluebird made a nest in the old dead tree trunk.

It would need to be quite deep, allowing for a decent sized back cushion in order to be comfortable enough to lounge/read on for a period of time. A comfortable sofa sans cushion is about 28" deep. Allowing for a 4" cushion gives you 24" to sit on. I don't think your plan would accommodate that.

It will be one of many as I also have an east facing loft and a southwest tower room in which to lounge and enjoy the natural amenities that the seashore has to offer. This is a retirement home and that window looks north at some pretty big live oak trees and big blue skies, and I am sure the birds will be putting on a show. thanks for the perspective. 😊

I have three window seats in my house as well, and imagined my children curling up in the spot to read a book. One pair, flanking cabinetry in my family room is 22” deep, which is too narrow to comfortably spread out and read on, and the other is 25” deep. They have custom made, great quality cushions which I felt were important for comfort as well as appearances.
Despite this and beautiful views, the only users in the past twenty years have been my cats!
I’m sure your room will be beautiful, and the seats will add a nice architectural feature, but I found that
they just can’t compete in comfort with a great club chair or even the sofa.

I am surely rethinking my design for that area. Cozy club chair/ottoman may be the ticket. Either way the cats will appreciate the effort!

My experience has been the same as AC M. I also have 3 window seats and no one ever sits on them. They mostly function as a place to drop stuff :(

My Stickley recliner is more comfortable than any window seat I have encountered.

WIndow seats is one of those pervasive fantasies embedded in brains from reading too much Agatha Christie and the like. They originate from old actual masonry structures needing to be FEET thick, so window openings were too. And they were drafty, so you get the little girl hiding behind the thick velvet drapes, overhearing the maguffin to set things in motion. Quaint and charming and realities of modern life don't align. Reality is that a comfy chair with a laptop stand and USB outlet close by is the new modern standard. Books, sadly, are electronic objects in many households.