From a post by Peter Jackson in Facebook:
It is only at the end of a shoot that you finally get the chance to sit down and have a look at the film you have made. Recently Fran, Phil and I did just this when we watched for the first time an early cut of the first movie – and a large chunk of the second. We were really pleased with the way the story was coming together, in particular, the strength of the characters and the cast who have brought them to life. All of which gave rise to a simple question: do we take this chance to tell more of the tale? And the answer from our perspective as the filmmakers, and as fans, was an unreserved ‘yes.’
We know how much of the story of Bilbo Baggins, the Wizard Gandalf, the Dwarves of Erebor, the rise of the Necromancer, and the Battle of Dol Guldur will remain untold if we do not take this chance. The richness of the story of The Hobbit, as well as some of the related material in the appendices of The Lord of the Rings, allows us to tell the full story of the adventures of Bilbo Baggins and the part he played in the sometimes dangerous, but at all times exciting, history of Middle-earth.
So, without further ado and on behalf of New Line Cinema, Warner Bros. Pictures, Metro-Goldwyn-Mayer, Wingnut Films, and the entire cast and crew of “The Hobbit” films, I’d like to announce that two films will become three.
It has been an unexpected journey indeed, and in the words of Professor Tolkien himself, “a tale that grew in the telling.”