Franklin had talked about the prospect of moving in with the Defenders with his parents.  They reacted as he thought they would, but they eventually saw his side of the issue.  He figured it was, in part, because both of his parents realized that there was nothing they could do to stop him from doing what he felt was best.

Sydney, the den mother, had been informed before Franklin arrived.  He had been given her e-mail address prior to leaving, in order to contact her about when he was going to arrive.  Of course, it wasn't guaranteed that he was moving in with them.  The visit was just to see how he vibed with the Defenders.  If they had problems with him being there, it wasn't going to happen. 

That being said, Franklin doubled-checked himself before knocking on the door to the main building.