Building a grave beneath a walkway
I would like the legal ruling on the following: We own a tomb which has become full and, as a result, would like to build a room beneath a path adjacent to the tomb with a 20 cm concrete ceiling. Please note that this room will be below ground level and people, other than us, will pass over it to bury their dead in nearby graves. Is it permissible to build such a room and to bury the previously interred bodies in it?
Please note that we cannot build a new grave on the existing field. If it is impermissible, what are we to do with the remains of the previously interred bodies?
Allah has honored man, dead and alive. In Surat al-Isra`, He says,
We have honored the sons of Adam; provided them with transport on land and sea; given them for sustenance things good and pure; and conferred upon them special favors, above a great part of Our creation.[Al-Israa`, 70]
The Prophet considered the violation of a dead body as tantamount to violating a living person; he said, "Breaking the bones of a deceased is tantamount to breaking the bones of a living person" [Recorded by Abu Dawud]. This hadith demonstrates that there is a certain sanctity for dead bodies just as there is for living people. Furthermore, the Prophet prohibited against treading on graves; he said, "It is better if I were to walk on hot coal, a sword or mend my shoes with my feet than to walk on a grave" [Recorded by Ibn Majah with a good ascription]. Abu Marthad al-Ghanwi narrated that the Prophet said, "Do not sit on graves and do not pray in their direction" [Muslim].
It is known that whatever leads to a prohibition is itself prohibited. Based on this, it is impermissible to build a grave beneath a walkway. Moreover, the road is public property and not just for the owner of the tomb; it is therefore impermissible for the latter to use the road for his own interests. As for the bodies which were previously interred, you can make multiple levels inside a single grave in such a manner that each body is buriedseparately.
Allah the Almighty knows best.