May one look into his neighbor’s property?
It is forbidden to look into another person’s property to see what he is doing. This applies even if there is an open window through which the person can see from. Due to this, it is obligatory for neighbors to create partitions to protect each other from the other staring in his property. Even in the event that a partition was not made the neighbors must beware as much as possible not to look into each other’s property. This law applies likewise to shared backyards or porches.
Looking into the property out of casualness: There are opinions that rule it is forbidden to stand by the opening of another person’s property and look inside even if he has no intent to discern the person’s actions. Rather one is to turn his face and not look in, while he is standing in front of the door to their property. However while one is walking there is no prohibition for one to causally see his friends property, as the prohibition is only for one to stop and stare inside.