Is it permissible to build a mosque or another building on land that was previously a cemetery?

It is not appropriate to allocate a place that has been endowed as a cemetery for another purpose as long as it is possible to use it for its original function.

Even if burials have not taken place there for a long time, it is proper to preserve the area as a graveyard. Unless there is a necessity, it is not correct to sell such a cemetery, build on it, or move the bones of the deceased to another cemetery for such purposes. (Ibn Abidin, Radd al-Muhtar, 5/58; 6/428) However, in cases where there is no alternative or public interest requires it, the cemetery may be relocated and the land can be used for purposes such as building a mosque.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu