Commento su Bava Metzia 10:3

הבית והעליה של שנים – the house belongs to this one and the upper story to the other.

If a house (the bottom and an upper room belonging to two persons (each owning fell down, and the owner of the upper room told the owner of the house (the bottom to rebuild it, and he did not want to rebuild it, the owner of the upper room may rebuild the house below and live in it until the other repays him what he has spent.
Rabbi Judah says: “He would then [after being reimbursed] have been dwelling on his fellow’s property and he should [therefore] pay him rent. Rather the owner of the upper room should rebuild both the house below and the upper room and put a roof on the upper room, and live in the house below until the other repays him what he has spent.”
Mishnah three continues to deal with a situation in which one person owns the bottom story of a house and another owns the top story, and the house collapses. In the scenario in this mishnah the owner of the bottom story refuses to rebuild his share, thereby preventing the owner of the top story from rebuilding his share.
Mishnah four deals with a case similar to that in mishnah three, except instead of a house here one person owns an underground olive press and another person owns a garden planted above the olive press, and the olive press collapses.
In the scenario in our mishnah after the entire house fell, the owner of the upper room wants to rebuild it so that he can return to live there, but the owner of the bottom story refuses to rebuild his share, thereby preventing the former from rebuilding his share. According to the opinion in section one, the owner of the upper room may rebuild the bottom story and live in it until its proper owner repays him for his expenses. Once the bottom story is rebuilt the owner of the upper room will be able to rebuild his share and live there.
According to Rabbi Judah, in this scenario it would turn out that the owner of the upper room had lived on the property of the owner of the bottom story without paying him rent. After all, at this point the upper room no longer existed and its owner therefore could not have lived there. Although the owner of the lower story should have rebuilt the house, he was not obligated to rebuild it and let the owner of the lower story live there for free.
Rather the owner of the upper room should rebuild the entire house, including the upper room, but live in the bottom story until it owner repays him for his costs. In this way the owner of the lower story could not claim from him rent, since while he was living there he had the upper room at his disposal.

אמר בעל עליה לבעל הבית לבנות – the wall and the lower ceiling that are upon him to built

בונה בית – and the lower ceiling that is upon it, and dwells in the house until he (i.e., the person who lives in the lower area of the house) compensates him for his expenditures, and afterwards, he leaves and builds his upper story.

אמר רבי יהודה אף זה דר בתוך של חבירו – if this is the case, the owner of this upper story in this case compensates him for what he has spent, it is found that he is living all these days in [the section] of his fellow, and even though he is not without that, he would not have built it; nevertheless this one is benefitting, for were it not for this house, he would not have had a place to live there. And he holds that this one benefits and this one is not lacking, he is liable. But he builds it all.

ומקרה את העליה – above him and all that is necessary for it.

ויושב בבית – [in] the lower [house], for he would have that this one doesn’t benefit, for the upper story was ready for him to live in it and the other person is not lacking anything, for had he not built it, it would not be appropriate for him.