A lot of people have had issues with this and it is a known design flaw on VW's part as mud and slime collects behind the wheel arch causing the rust. VW do a 12 year warranty on rust as long as it is not caused by the owner modding/ not looking after the car.
Take it to your VW garage and they will inspect it/photograph it, and take your details and they should get back to you to let you know if you have been approved or not for repair work. some do, some don't.
I actually need to take mine due to minor rusting in the same area, and also on the rear door lock on the bodywork.