I got this question recently:
“I wrote a screenplay based on an existing franchise. I’m in the process of pending the permissions with the right attorney. If I am granted the rights to sell my script, and that’s a big “if”. What is the right way to state it in my query letter, if it’s what I need to do?”
First, hopefully you realize writing a script based on an existing franchise is not recommended. The chances of getting the rights to it are very slim and then your only chance is to sell it outright to the owner of that franchise or use it as a writing sample. In the feature realm I’ve never heard of a new writer using a spec script that’s based on an existing franchise so it might scream amateur. I’m not saying it’s never been done but I’ve never heard of anyone doing it. While with television writing the opposite is true and if you’re trying to break into television writing you will write spec scripts based on currently popular television shows.
But assuming you do get the rights to the franchise (make sure a good entertainment lawyer draws up the paper work because you’ll want this contract to be air tight) I would simply state in your query letter in as clear terms as possible probably in the first paragraph where you pitch your script’s logline. At that point you’ll mention that your script is based on an existing franchise and that you have the legal right to sell your script based on that franchise. Be clear and concise in your query letter. Make sure you talk with your entertainment lawyer and understand the particulars of your deal because the details will be very important as this project moves forward.
Check out this post as it covers some of the same ground: Writing a spec screenplay sequel.
Also, this post might be helpful, too: Writing a screenplay based on a book, comic, video game, or toy.