About ten years ago my neighbor and I added a floor to our one story house. We built independently. He built an outside wall on all four sides of his addition and I did the same. Thus each one of us owns all of his walls by himself.
This year we are both adding another floor. When we looked into building this year, we discovered that my neighbor’s wall on the second floor was built totally on my property. It turns out that it was in a sense a blessing in disguise because in order to get a permit to build this time, I was required to build a reinforced wall, which requires the support of a strong wall which has beams all the way down to the ground. Had he built his wall where it should have been built I would not have had the support of his wall. It is only because he built on my property that I have his very strong wall to rest my new reinforced wall on. Do I have to pay my neighbor anything for using his wall?
As we mentioned in the past, we have a rule of zeh nehene vezeh lo chosier potur, that one who benefits from another person who loses nothing thereby does not have to pay anything. Thus it would seem that you should have no liability at all.
However, we find in the Gemara a similar situation where one does have to pay. Once we understand why one has to pay in the Gemara’s situation we will be able to address your question.
The case in the Gemara (Bava Basra 5A) concerns a wall built between the yards of two neighbors. The din is that each neighbor can force the other to share in the cost of a four amo (about two meters) high wall which is to be built half on each neighbor’s property. The reason they can force each other is because it is proper to build such a wall in order to prevent each one from violating the other’s privacy.
The Mishna continues that if one wishes to build a higher wall he cannot force his neighbor to share in the additional cost because it is not mandatory. However, if at some later time the neighbor acts in a manner that indicates that he too wishes to use the added height of the wall, he is required to share in the cost of the construction. The Rishonim question why he must share in the cost since the wall was there anyway so it would seem that this is an application of the rule that zeh nehene vezeh lo chosier potur.
The Rishonim suggest two approaches. The approach of the Magid Mishne (Shechainim 3, 4) and the Nemukei Yosef (Bava Basra 3A) is that when the first neighbor added to the height of the wall, he wanted to share the wall with his neighbor. Therefore, at the outset, he placed half of the added height in the neighbor’s property (since half of the original lower four amos stood on the neighbor’s property) with the intention that if at some future time the neighbor will desire to use the wall, the neighbor can pay for half and acquire the half that stands on his property. Even though at the outset the second neighbor refused to participate in the construction, once he indicates that he wishes to use the added height of the wall we say that automatically he is taking up his neighbor’s offer to purchase half of the wall and therefore, he is required to pay half of the cost since he is buying ownership of the added height of the wall.
The approach of Tosafos (Bava Kama 20B and Bava Basra 5A) is very different. Tosafos does not view the neighbor as buying ownership of the wall but rather suggest that this is an exception to the rule that zeh nehene vezeh lo chosier potur, since the second neighbor indicated that he is willing to spend money on a project which includes use of the added height of the wall.
These seem to be two totally different approaches to explain why the second neighbor must share in the cost: the first approach is that it is a sale and the second that he is paying for benefiting from the neighbor’s wall. This is how many Acharonim including the Ketsos (158, 6) understood. Furthermore, there are Acharonim such as the Yam Shel Shlomo (Bava Kama 2, 16) who don’t accept the approach of Tosafos because it is a chiddush that has no inherent support in the Gemoro. It is only suggested because it is way to answer a difficult question. However, if there is an approach that does not require such chiddushim we should rather follow that approach.
However, each approach is difficult. The approach of the Magid Mishne is difficult because the Gemara (Bava Kama 20B) discusses an equivalent case (where the outside neighbor rather than the side-by-side neighbor built the wall) and the Gemara understood that the payment is for benefit and not for purchasing ownership. Other Acharonim question the approach of Tosafos because the Gemara says that the second neighbor must pay half of the cost. If he is only paying for use of his neighbor’s wall he should pay the value of using someone else’s wall and not half of the cost of the wall since he does not attain ownership.
Rabbi Akiva Eiger (printed in Zichron Yehuda) and Rav Shimon Shkop (Bava Basra 4, 3) both suggest that the two approaches complement each other and we accept both approaches. If the second neighbor would not be required to pay for using his neighbor’s wall he would never be interested in purchasing ownership of the wall. It is only because he anyway would have to pay for use of the wall that he is encouraged to already buy ownership of half of the wall.
It would seem that in your situation if we follow the approach of the Magid Mishna and Nemukei Yosef you do not need to pay at all since your neighbor did not build with any intention to share the wall with you. Ten years ago he built his own wall and you built your own wall. Therefore it would follow that you shouldn’t have to pay at all for using his wall since he is losing nothing.
It is only according to the approach of Tosafos that you would have to pay. According to the approach of Rabbi Akiva Eiger and Rav Shimon you would not need to pay half, but an amount for use that beis din would have to determine.
However, in your situation there is another very important fact to consider.
The Ramo (360, 1) rules, based upon several Rishonim including the Rashba, that you have the right to ask your neighbor to tear down his wall completely since he built it on your property. There are Acharonim (See Pischei Teshuva 360, 1) who say that this is a matter of dispute. However, even according to the lenient opinion you can ask your neighbor to pay you for your land which he (mistakenly) stole by building his wall on your property. Therefore, it would seem proper for you to make a deal with your neighbor that you will allow him to leave his wall without paying, in exchange for him allowing you to use his wall without paying him for its use.