I'm building a 20 x 21 family room and I would like to know where to place the two heat registers I plan on putting in.

Under the windows
, but be aware : when your furnace was installed was the size calculated and oversized for the over 200 addition Sq.F that was to be added if not you may the existing furnace to be to small and the ac if you have one,, good luck

Yep, always on the outside wall under the windows.

I guess I should`ve said,, But I just assumed all windows are in outside walls, but I guess people could put windows on inside walls if they want to,, lol :)

Just mentioned the outside wall in case they were building some sort of weird room without windows,, he, he, that's my excuse and Im sticking to it,