The Mechaber (152:1) writes that one may not demolish an existing Bais Haknesses in order to build another one. We are concerned that something unexpected will stop them from building the new Bais Haknesses. The new Bais Haknesses should be built first and only afterwards can the old one be demolished. The entire building must be finished.
This is true even if all the money and materials needed for the building have been secured. We are still concerned that an opportunity of pidyon shvuyim – redeeming a captive – may suddenly happen, where they would be obligated to give all of that money towards that mitzvah. There is a discussion amongst the poskim if this halacha is from the Torah or only M’Derabanan.
Even if they want to use the stones from the old Bais Haknesses for the new one, they are not permitted to do so and must first build and entirely complete the new Bais Haknesses. The Rema adds that this also applies if they only want to demolish one wall in order to make more room in the Bais Haknesses. First the new wall should be built and only afterwards is the old wall demolished. If it is not possible to build the new wall first, the Biur Halacha cites different opinions if they may first demolish the old wall.
The same applies if they are only renting a place as a Bais Haknesses and they want to rent a new place. They may stop renting the old Bais Haknesses only after they have already started renting the new one.
The Mishnah Berurah brings a machlokes between the Taz and the Magen Avrohom if these halachos apply even when there is another Bais Haknesses in the city where they can daven in the meantime. The Taz holds that in such a case it would be permissible to demolish the old Bais Haknesses first. The Magen Avrohom disagrees and holds that even in this case it would be prohibited.
The Machatzis Hashekel explains the different opinions. The Taz goes with the opinion of the Ran that our concern is that if they demolish the Bais Haknesses first, it may take them a long time until they build the new one and they will have nowhere to daven in between. If there is another Bais Haknesses in the city this is not a concern. However the Magen Avrohom understands that the primary concern is that they will never rebuild it, therefore even if there is another Bais Haknesses in the city it is still prohibited to demolish the old Bais Haknesses until the new one is built. The Biur Halacha rules like the opinion of the Taz that it is permissible. However if the other Bais Haknesses in the city doesn’t have room for everyone, then according to all opinions a new Bais Haknesses must be built first.
It is questionable what the halacha is in a city where different groups have their own Bais Haknesses in which they daven. Even if there are other Batei Knessios in the city, it may still be considered that there is no other Bais Haknesses. The same question applies if the other Bais Haknesses is very far away.
The Mechaber ends off that if the Bais Haknesses is on the verge of collapsing then it should be demolished right away and a new one should be built immediately.1
Prepared by R’ Avrohom Yehoshua Ziskind
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1Sources: Mishnah Berurah, Biur Halacha, Dirshu, Piskei Teshuvos