We are trying to build a new masjid next to our old Masjid. Once the new Masjid is build, what should we do with the old one? We were told that once a buliding is a masjid it has to stay that way or build a new masjid on the same ground as the old one.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

If a person builds a mosque and allows people to pray in it, then it becomes an endowment and, in principle, it is forbidden to sell it, or replace it with something else, or transfer it to another place. Therefore, if it is possible to benefit from the old mosque –after finishing the building of the new mosque –by keeping it as a prayer place for the five daily obligatory prayer (except the Friday prayer) and the neighbours would pray in it, or that it will be assigned for teaching the Quran in it along with the performance of the obligatory prayers, then it should be kept as it is. However, if it is not possible to benefit from it as a mosque and it was abandoned and became useless, then in this case, it is permissible to transfer it to another place by selling it and building another mosque in another place. Imaam Ahmad was asked about a mosque which was ruined whether its land can be sold and the money will be spent on building another mosque, and he replied: “If there are no neighbours around this mosque and there is no one who prays in it, then it is permissible for it to be sold and the money will be spent on (building) another mosque. 

For more benefit, please refer to Fatwa 87548.

Allaah Knows best.