Raising window heads in conventional platform or ballon framed stud walls is common and easy to do with prior planning befor constructing the walls. The normally required structural header is simply raised to accomodate the desired window head. This is sometimes referred to as "flush framing". Every builder knows what it is.

Of course, if you are considering this for very wide openings, such as closely multiple ganged windows or doors, more detailed structural design of the header and its connections may be needed.

For a single 3' wide (or so) double hung or other type of window, however, it's quite simple and common.

Good luck on your project.

Here is an interior view illustrating flush framing for the living room windows:

Nice! I am glad to know this option is available. I will rethink my window design for a particular wall. The room is our breakfast room/morning room/kitchen which overlooks the pond in the back. This side of the house faces north. Our property is in South Carolina so facing north is not necessarily a bad thing in this region, but it is still good to know I can raise the windows to bring in more light.

Thank you for your responses.