The mishna[1] discusses a type of wall built in order to divide property shared by two people. The gemara[2] discusses whether the two have to agree to a wall, or whether once they agree to divide the property, each can demand of the other to erect a wall between the sections. The gemara says that it depends on whether we consider the fact that one neighbor can see what the other is doing on his property as a damaging situation. We accept the opinion that visual intrusion into another’s privacy does constitute damage, and thus one neighbor has a unilateral right to have a wall erected.[3]
In many questions of rights between neighbors, there is a concept of chazaka: if one side exercised a certain usage of his property that impacts on his neighbor without the neighbor protesting, he can continue doing so. (The logic, parameters, and opinions on the matter are beyond our scope.) However, the Rambam[4] rules that the absence of walls between properties does not create a chazaka of living without walls, and either neighbor can later demand that one be erected.
There are two explanations for this halacha. Usually, a chazaka occurs when one side takes an active step that would usually trigger a protest if the other party opposed it, so that the other party’s silence is equivalent to agreement. The lack of a fence is a passive situation, and so the absence of a demand to erect one is not proof that such a demand would not be made in the future.[5] Another distinction is that in the standard case of chazaka, one party does something from which he benefits while the other stands to lose. In such a case, we reason that if the potential loser in the new situation is quiet, he must have gone along with the change. However, in this circumstance, in which each neighbor is the potential gainer and the potential loser from the lack of a wall, we consider the possibility that one did not feel a need to initiate steps to put up the wall, which his neighbor could also do. Therefore, he can exercise his rights in the future.[6]
Either way, in your case, you would have to demonstrate that there was an outright agreement (even an oral one[7]) by your neighbor or one of the previous owners to waive his right to a wall. The Rama[8] accepts the opinion in the Rishonim that one’s right to a wall is valid even in places where the practice is not to have such walls.
Normally, both sides have to share equally in the expenses and in ceding space from their properties for the wall’s location.[9] The matter becomes complicated when there is disagreement regarding the thickness and quality of the wall and the cost. In general, the wall between residential yards should be four amot (approximately six feet) high so that it effectively obstructs the view.[10] Similarly, the wall must be made of a material that will obstruct the view. However, one can force his neighbor to pay only for the level of construction that local practice – or, in its absence, a beit din or an expert – considers a standard wall.[11]
There are additional details that could affect the halacha in a major or a minor way that might come up in an adjudication over this point of contention, were you to need to engage in one. However, we trust that the general information we have provided gives you the basic Torah perspective about the demand for privacy and the financial ramifications of that right. We hope this background will help you work out an amicable resolution with your neighbor that takes into consideration the desires and concerns of each of you.