You have J.K. Rowling's permission to do so!
Rowling has given fans permission to write sequels to the Harry Potter books, as long as the material stays online only.
Rowling's lawyers confirmed she is happy for spin-offs to be published online as long as the publications are not sold and it is made clear she was not involved in the stories.She also requested the follow-ups do not contain pornography or racism.
This is great news for those of us who write Harry Potter fan fiction! So get started already... and share it with the world in The Three Broomsticks!