Siman 152

One may not demolish a beit knesset in order to build another in its place or in another place. And it is not only necessary to state that if the demolition is not for the purpose of building another—for example, if one builds another beit knesset in another place from new wood and stones—that it is obvious that one does not demolish the old one until they build the new one. Rather, even if one needs to demolish the old one first in order to build another in its place, or even if one builds in another place but needs to demolish this one to take wood and stones from it to build the other—nevertheless it is forbidden (Bach), for we fear perhaps an accident will occur to them such that they will not build the other. Rather, they build the other first, and afterwards they demolish the old one. And even if they did not want to demolish all of it, but only to demolish one wall to expand it—the law is also thus, that they build the new wall first and afterwards they demolish the old one. And this matter is permitted even in large cities. And even though selling it entirely is forbidden in large cities, as I wrote in siman 153—this is because perhaps it is not pleasing to everyone. But to build another new one—it is certainly pleasing to them (Magen Avraham subsection 2).

Some say that even if there are two batei knesiot in a city or more – it is forbidden to demolish one of them until they build another in its place (ibid. subsection 1). And some say that in such a case it is permitted. And the reason: for we do not fear that an accident will occur such that they will never build it, as this is an uncommon concern. Rather, the concern is lest an accident occur and they prolong the construction for a long time, and in the meantime they will have no place to pray. Therefore, when there is another beit knesset in the city where there is a place to pray – we have no concern (Taz subsection 1, and Bach, and Eliyah Rabbah subsection 1). And since the majority of the poskim hold so – one should be lenient in such a case. (For in the Rashba and Ran and Nimukei Yosef it is explicitly permitted in such a case, only the Beit Yosef wrote that according to the Tosafot and the Tur it is not so. And they explained at length that even according to the Tosafot and the Tur it is so.)

And we say in the Gemara (at the beginning of Bava Batra) that this prohibition only applies when there is no structural flaw in the old one. But when we see a structural flaw in it—it is permitted in any case. That is to say: specifically when the old one is still strong and there is no concern of damage, then it is forbidden to demolish it until they build the new one. But if its foundations have crumbled, or its walls are leaning toward falling, or there is danger in the roof or in the beams—they demolish it immediately. They only begin to build the new one by day and by night if possible, and they hasten its construction so that it is not drawn out for long. And in the Gemara there, it is brought regarding an incident of this sort, that one of the sages brought his bed in there so that they would hasten its construction. (And see Magen Avraham subsection 4, that the intention is near the synagogue, see there. And it is not compelling, as the Eliyah Rabbah wrote in subsection 4, that for the need of the synagogue—it is even permitted in the synagogue. And also what he wrote in subsection 5 against the Bach follows his own reasoning in subsection 1. And it is not so, as I wrote in section 2. And also the intention of the Bach is not as he wrote, but rather when the person stands in the middle of the building. And the Machatzit HaShekel already noted this, see there.)

Our teacher, the Rema, wrote that it is forbidden to take stones from an old beit din to build a new one. Thus far his words. That is to say: that which was explained, that it is forbidden to demolish an old beit knesset—this is even when one needs the stones for the construction of the new beit knesset, and we have already explained this in section 1. And furthermore it teaches us that even if he already began to build the new one, and stands in the middle of the construction—it is forbidden. (Eliyah Rabbah and Magen Avraham subsection 5. However, that which the Bach wrote to permit in such a case—this is when they have another beit knesset, and as I wrote in section 2. And the Magen Avraham understood in him another intention, and it is not so as I wrote. Examine it and you will find it simple.)

He further wrote that it is forbidden to demolish anything from a beit knesset unless he does so on the condition of rebuilding. Thus far his words. The explanation of his words: For it is stated in the Sifrei on Parshat "Re'eh": From where do we know that one who shatters a stone from the Heikhal, from the Azarah, or from the Altar violates a negative commandment? The verse teaches: "You shall not do so to Hashem your God." And since the beit knesset is a "minor sanctuary" – it is forbidden to shatter or demolish any attached thing within it. However, if his intention is to demolish on the condition of rebuilding, and through this demolition he improves – it is permitted, for this is not considered demolition. There are those who forbid making even a small hole in the wall to insert a piece of wood or the iron of a shelf (Taz siman 151 subsection 3). And there are those who permit (Eliyah Rabbah subsection 7), for anything that is not by way of destruction is permitted, as the Rambam wrote in the first chapter of Beit HaBechirah even regarding the Beit HaMikdash, and all the more so regarding a beit knesset. This is the primary view, and such is the common custom. And a beit knesset and a beit midrash have the same law in all the laws of this siman. (And from Makkot 22a it is proven that even a detached object is forbidden.)