Are you replacing a window or framing a new hole?
This is "window seat" with an asterisk.
It's really going to be a giant bookshelf w/ a nice flat spot to lay down on right next to the existing double hung window.
I am not pushing the window out as in a traditional window seat, this is a compromise
Totally removable and portable should the next person who lives here not like it.
The intention of this room was always to turn it into a library, and a cozy spot near the window would be a nice place to read