If you'd like to send a letter to Emma Watson, don't try to find out her private address. That's too difficult, and I'm not sure if Emma would really love your private post ...
Instead, we recommend using this address. Letters sent to this address arrive at the letterbox of the Leavesden Studios which appear to have the best contact to Emma.
c/o LEAVESDEN STUDIOS
Harry Potter Production Office
P. O. Box 3000
Leavesden, Herts WD25 7LT
If you want your letter to be forwarded to Emma, you should try to write in a serious style and your letter shouldn't be one of those "Just 4 Fun"-letters. Emma's assistants receive too many of that kind ...