For your own safety you should at least ask the architect who designed your garage about how much you can safely store there. He may already have those joists at near max. load with how he designed it.
What does the garage have to do with the house? They have separate joists as they are completely different structures. I think your confused. The garage joists were oversized a LOT so there's no way in hell weight will ever be an issue in the new garage. The new garage attic space is already floored so it couldn't even be the subject of this thread. -- IF YOU FIND ANY MISTAKES IN MY WORK...Please consider that they are there for a purpose. I try to please everyone and there is always someone looking for mistakes!