May one look into his neighbor’s property?
It is forbidden to look into another person’s property to see his actions and dealings, without his permission and knowledge. This law applies even to a backyard and porch and goes without saying that it applies to a house. This applies even the owner sees him looking inside and does not protest, as perhaps he is simply embarrassed to tell him. [Even if there is an open window through which the person can see from, it is still forbidden to stand there and look into another’s property.]
Neighbors with shared property: This law applies even to shared backyards or porches, that one neighbor may not look into the other neighbor’s dealings and actions in his section. Due to this, it is obligatory for neighbors to create partitions to protect each other from the other staring in his property, as explained below. Even in the event that a partition was not made, the neighbors must beware as much as possible not to look at what the other person is doing in his property.
Looking at the property without intent to look inside: There are opinions that rule that one must beware not to stand by the (opening) of another person’s property and look at it, even if it is a quick glance and he has no intent to discern the person’s actions. Rather, one is to turn his face and not look in front of him when standing in front of the (door to their) property. This applies whether the property is a house or a courtyard.
Looking into someone’s property while walking down the street: (There is no prohibition for one to causally look at another’s property while he is walking outside in public. However, it is forbidden for one to stop and stare inside another’s property even while walking.)
Ayin Hara: It is forbidden to look into another person’s property [even if they are not present there] if there is possibility that the evil eye can damage their property. Thus, for example, one may stand over the field of his friend to stare at it at the time that its crops have grown full.
It is forbidden to look into another person’s property without his permission, whether it is his house, porch or backyard. This applies even for neighbors who share properties together. One may not stand and face the opening of one’s property, appearing as if he is trying to look inside, even if he has no intent to look into it and see what the person is doing. One may causally look at another’s property while he is walking outside in public, however, it is forbidden for him to stop and stare inside another’s property even while walking.
May one look into another person’s property if there is no one home?
It is forbidden to look into another person’s property even if they are not present.
May one look inside someone’s house when they open the door for him, but have not yet let him inside?
It is forbidden to look inside to discern the person’s action and dealings, as perhaps the person who opened the door does not want him to look, and simply opened it to talk to him and see what he wants. However, if one is not intending to look at the person’s actions in his home and is simply looking forward and sees the inside of the house in view, then if the door was opened by an adult, it is permitted to look inside. If however the door was opened by a child then seemingly one should avoid staring into the home and is rather to turn his face to the side until an adult greets him.
May one look into other rooms of the house of a host whom he is guest by?
Seemingly it is forbidden to do so, as the host did not give him permission to use those rooms, and perhaps he desires his privacy to be kept there. This is no different than the rules of privacy delegated to neighbors who share courtyards.
 Admur Nizkei Mamon 11-13; For a thorough summary of the laws of Hezek Reiyah-see Sefer Beis Vaad Lachachamim – Nezek Shecheinim p. 134
 Admur Nizkei Mamon 11; Rama 154/3; Michaber 357/1 “Hezek Reiyah”; Tur 157; Rosh Baba Basra 1; Baba Basra 2b
The reason: It is forbidden to look into another person’s property without his permission to see what he is doing as perhaps he does not desire that other people know of his actions and dealings. [Admur ibid] Doing so can cause the person damage, as the invasion on his privacy will stop him from doing certain actions in his property. [Smeh 378/4] This applies even if there is no worry of Ayin Hara as a result of this looking. If however there is worry that looking into the property will cause an Ayin Hara, then it is certainly forbidden to look into the property as doing so will cause it damage. [Admur ibid; Michaber 378/5; Baba Metiza 107a; Baba Basra 2b See Smeh 378/4 that the Ayin Hara itself causes damage]
The source: The Sages stated on the verse “And Balaam raised his eyes and saw Israel dwelling according to their tribes” that he saw that the openings of their tents are not opposite each other. Balaam then stated that it is thus befitting that the Divine presence rests on them. [Admur Nizkei Mamon 12; Baba Basra 60a]
 Admur ibid and Rama 154/7 rule that even if an owner consented for a neighbor to open his window and thus be in full view of his courtyard, it still does not permit the neighbor to stand and look into his property, as the permission was only granted for the sake of the neighbor having light and not because he does not mind people staring into his property. It is thus forbidden for him to look. [Admur ibid; Rama ibid; Rashba 2/1] Accordingly, it is forbidden to look into another person’s property even if the window or door is open, as it is simply opened to allow in light or fresh air and not because one does not mind others looking inside.
 Admur Nizkei Mamon 13; Rama 154/3
 Nimueki Yosef Baba Basra 3/32
 The reason: As although he has no intent to look inside [and thus it should not be prohibited due to Hezek Reiyah-Smeh 154/14], nevertheless when standing in front of the property and looking forward it appears to his friend that he is trying to look into his property to know of his dealings, and he will thus be viewed as a robber, being he has no business to be looking at it. [Admur ibid; Rama ibid] This does not mean that the person will appear as if he is spying in order to steal objects from the home but rather that the mere fact that he is looking inside someone else’s property to know what they are doing is itself considered a form of stealing. [See Smeh 154/14]
 Admur Nizkei Mamon 13 in parentheses; Based on Admur 12 and Baba Basra 60a that people must be modest in their home to protect themselves from damage from the public seeing their actions.
 The reason: As it is not possible to beware from seeing others properties while walking, and each person in their property must be careful and do things in enough privacy in a way that a mere glance from an outsider will not discover his actions. [Admur ibid in parentehses]
 Admur Nizkei Mamon 11; Michaber 378/5; Baba Metiza 107a; Baba Basra 2b
 See Q&A!
 The reason: As perhaps the owner does not want people knowing what he has in his home and the things that he is involved in. In addition, if the home is nice and fancy there is worry that Ayin Hara can damage it.
 The reason: As the adult who lives in the home has given him permission to do so by the mere fact he opened the door for him. Likewise he will not be looked at as a Ganav being that he is there for a purpose and does not appear as if he simply wants to look in.
 The reason: As no adult in the home has given him permission to look inside, and perhaps they do not want him to see their actions inside the home. Now, although in such a case the person will not be viewed as trying to spy the home even if he faces the home and looks casually into it, being that he is there for some purpose, nevertheless people are not comfortable with others looking into their home when it is opened without their knowledge, and hence it is to be avoided even with a casual look.