What is the ruling of the Shari'ah regarding the filling in of an old cemetery where no one has been buried for over 40 years, and building an extension for a mosque over it?

It is not permissible to build mosques over graves unless the cemetery has fallen into disuse and its inhabitants have decomposed. The reference for this is to the experts in the field. If they inform that the dead decompose in the graveyard within forty years, then it is permissible to utilize it for building, cultivation, or filling it in and walking over it. Otherwise, it is not permissible.