Recently you answered a question concerning someone (who was a sofer) who built a small apartment on the common area which belongs to his entire building and the neighbors allowed him to use the apartment for free. Furthermore, you said that if the neighbors later want the apartment they have to pay the sofer for the building. We have a similar situation but we don’t want to allow the builder to use the apartment for free. Can we tell the builder that we want to buy the apartment but we will pay for it by reducing the amount that is due him for the building by the amount that it costs to rent a similar apartment in our neighborhood? In this manner, each month we will reduce the amount we owe him instead of paying in advance.
Your question is similar to a question which was asked to the Maharach Ohr Zorua (son of the Ohr Zorua, res. 257). In his situation, one of two partners in a house improved the house by adding extra rooms onto the existing house that was commonly owned. The one who improved the property then demanded that his partner, who did not raise an objection when he saw him adding rooms, pay his portion of the expense. The partner replied that his silence was only acquiescing for him to build, but he never agreed to pay for the improvements. The Maharach sided with the defendant, and further ruled that he had the right to tell his partner that he can use the added rooms as payment for his share of the building costs.
Thus, it would seem that we have a clear answer in this precedent to our question. However, it is important to analyze the basis for his decision. He cites as proof a situation which is discussed in the Mishna (Bava Metseyo 117A).
In the Mishna’s situation, a two story building collapsed and the owner of the second story apartment wanted to rebuild but the owner of the ground floor apartment refused, thereby preventing the owner of the second floor from rebuilding. The authoritative opinion of the Chachomim is that the owner of the second story has the right to rebuild the ground floor apartment and live there until the owner of the first floor pays his expenses. The Maharach understood that the Mishna means that the amount the owner of the ground floor owes is reduced by the rent the owner of the second floor would have had to pay otherwise.
However, this approach is very difficult because the Gemara (Bava Kama 20B) clearly indicates that the owner of the second floor does not have to pay anything for living in the ground floor apartment and in fact this is the unequivocal ruling of the Shulchan Aruch (164, 5). Thus, the proof of the Maharach does not stand up halachically.
It is very important to note that this does not prove that the opposite is true, since the Gemara (Bava Kama 20B) says there is a special reason why the second floor owner may live for free in the ground floor apartment in the interim. The reason the Gemara gives is that one of the obligations of the ground floor owner is to enable the second floor owner to live in the building. Thus, if he prevents the second floor owner from rebuilding his own apartment because he doesn’t build his first floor, he must allow the second floor owner to rebuild the first floor for his own use. Since this is a special obligation, we cannot prove that in your case you cannot repay the builder by reducing your debt to him by the cost of renting the apartment he built.
The Nesivos (375, 3) discusses a similar case to yours, where a person built on another person’s property and the owner of the property either indicated by his actions or common sense dictated that he approved of the building. The question of the Nesivos was exactly yours: whether the owners can pay for their share in the building by allowing the builder to live there without charge, or if they needed to pay the entire cost right away. The Nesivos wanted to rule that the owners must pay immediately one lump sum for the entire amount that they owed. His argument was that since they owed the cost of building there is no reason to allow them to delay and also for allowing them to repay in installments since people prefer to be paid in one lump sum. According to the Nesivos’ approach, you could not do what you asked.
The Nesivos backed away from part of his ruling since it is clear from the Rosh (Bava Metsiyo 8, 23) that one cannot force the owner to pay for the improvements that he made on his own initiative, not immediately and not in installments. In fact, in two places (the above and in a responsa cited by the Tur in siman 375) the Rosh rules that what the one who improved the property can do is just to use the property until he is paid. The Nesivos writes that this is the way to pressure the owner to pay for the improvements. Obviously, the Nesivos understood the Rosh as intending that the one who made the improvements may live there without reducing the amount owed to him at all, since if the amount owed is reduced, living there will not pressure the property owner to pay. This is exactly like in the Gemoro where the owner of the second floor lives there for free.
There are two ways that one can explain why the one who improved the property may use it free of charge. One explanation is that it is his until the owner pays him. The Rosh clearly (Bava Kama 2, 6) indicates that this is his opinion. Another approach is that even though the property owner immediately acquires the improvements, still the builder may use them for free until he is paid as a means of pressuring the owner to pay.
The Chazon Ish (Bava Basra 2, 6 bezman) also understands that the one who improved the property may live there for free. He writes that the same reasoning which the Gemara gave to justify Chazal’s allowance of the second floor owner’s living for free, explains why the one who improved can use the additions he made for free, namely, because the property is subordinated to the one who improved it until he is paid in full.
We should point out that in general one who purchases anything cannot force the seller to accept credit if it wasn’t agreed to initially, since the seller doesn’t have to be the banker of his customer. Thus, in the case of an ordinary sale the Nesivos (190, 7) rules that if the customer pushes off the seller, the seller has the right to cancel the sale.
In conclusion: If you wish to acquire full ownership of the apartment you will have to pay the builder, and you cannot reduce the price by the amount of rent that the builder saved by living in the apartment that he built. The builder can use the apartment he built without paying rent until you pay him in full for building it.