Except Rowling and other authors give tacit permission to carry on with the fanfic by very carefully looking the other way. The ones that don't approve are obvious because the most basic way to publically disapprove is to have your lawyer send a quick letter to FFN, who hastily pull down the entire section. Anne Rice has, Raymond Feist has, Robin Hobb has, and quite a few others.

A smaller entity without lawyers - like a fanfic writer - doesn't have such tacit endorsements or denouncements as part of the public record. So you bloody well ask, because not doing so would make you a douche.

 

One could argue that most of the Independent!Harry fics out there are Power of Time and Years of Rebellion fanfics. I personally don't see a problem with that, so long as proper credit is given to uncommon concepts.

Pushing an existing story into a different direction than what the original author intended, possibly even reposting the whole thing with slight modifications, that's another kettle of fish entirely. Unless the original author truly doesn't give a fuck.

 

Taking a concept and expanding upon it is completely different to taking an idea and rewriting it to fit your imagination. Taking a concept is akin to seeing a canoe and taking the basics of it to build yourself a longboat. Taking an idea is like seeing the same canoe and building your own, but adding stabilisers to the sides.

The latter may not have been all that bad of an idea in terms of early seafaring, but when it comes to literature it's a very sorely contested subject.