Derivative patterns?

[flutterbychild.livejournal.com]

I want to make an American Girl doll dress, based off of a pattern I found on Ravelry for a baby dress. At what point can I claim the pattern as my own and not simply a modification of the original pattern? What I want to do (if I can figure out how) is add a hood and long- or 3/4 length sleeves. I know lots of people modify patterns and I don't mind saying that I did so, but I was just wondering...