Is there a part you could add to the 10k draft, a plot point you didn't explore or a character who needs some more page time? Are there sexytimes? It might feel like a cheap trick, but this could be an opportunity to write sex scenes to your heart's content. :-)

I'm with dont_hate_me, I have a hard time keeping stories short enough... but then again, if the draft feels complete, it might do the story a disservice to write more just to fill a word count. So I would say the integrity of the story comes first, and if you feel you can write 10k more words while honoring that, do it! And if not, write a new story, and if that one doesn't make it to 20k either, at least you have another story. <3 <3