When deciding custody, the courts in most states use the "best interests of a child" measurement to determine division of custody and visitation rights. When the parents live a significant distance from each other, whether the custodial or non-custodial parent moves, "the best interests of a child" are again brought into the equation along with other relocation factors. Unless the parents are able to come to an agreement outside of court, the court may decide to permit the move or not, and can order new custody or visitation agreements. Different states have different laws, so it's best to review the case with a legal counsel who is familiar with your state's law.
23 PA C.S. 5337 is Pennsylvania's custody relocation statute which requires any party seeking to move with minor children to get court approval or the other parent's permission prior to the relocation. A relocation is defined as any move that would "significantly impair the ability of the nonrelocating party to exercise custodial rights." A move that is only a few miles away would not count as a relocation. Procedurally, the party intending to relocate must give at least 60 days' notice, or notice as soon as possible, of the intended move. The party would include a counter-affidavit with the notice which allows the non-moving party to designate their position. If the move is contested a hearing on whether or not the relocation should be granted should be held prior to the move. In addition to addressing the 16 factors as to what's in the child's best interests required in any custody case, the moving party must also address 10 relocation factors. The moving party has the burden of proof to show relocation will serve the best interests of the child(ren) and that there is no improper motive in seeking to move.