All perfect praise be to Allah, The Lord of The Worlds, and my His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
In principle, a grave is a hole in the ground which keeps the smell of the corpse inside and keeps the animals away.
One dead person should be buried in each grave unless there is some necessity for burying more. As regards burying the dead in what is called "Fosqiahs", the dead is placed in an underground chamber, then its door is closed and he/she is left to transform therein. This method of burial is not approved by Sharia because the Sunnah has clarified how our Prophet(PBUH) used to bury dead Muslims.
Muslim scholars have forbidden exhuming a grave to bury another person in it because this violates the sanctity of the grave`s dweller and triggers the smell of the corpse, whether both were males, females or a male and a female, even if the latter were Mahrams(Prohibition by consanguinity), such as a person and his mother.
Therefore, burial in Fosqiahs isn`t lawful; rather, it is prohibited on basis of the afore restrictions. Moreover, it is worth reminding that the Jordanian Iftaa` Board has issued resolution No. (126) in which it prohibited burying more than one dead person in a single grave.
In addition, there is consensus amongst scholars that it is prohibited to exhume graves for doing so violates the sanctity of the dead. This is based on the Hadith where The Messenger of Allah (PBUH) said: " Breaking a dead man's bone is like breaking it when he is alive."{Abi Dawud}.
Furthermore, Allah, the Almighty, said, "We have honoured the sons of Adam"{Al-Isra`, 70}, and part of this honoring is not to exhume their graves nor violate their sanctity.
However, if the corpse has transformed into earth, it is permissible to exhume the grave and bury another in it. It is also permissible to establish a cemetery in the place of the above mentioned Fosqiahs, provided that it complies with the rules of Sharia in that regard. Al-Imam An-Nawawi said: "It isn`t permissible to bury a person in another`s grave unless its dweller has already transformed into earth, and nothing was left of his corpse; neither flesh nor bone."{Al-Majmou` Shareh Al-Mohazzab 5/284}.
In addition, the time that each corpse needs to reach a state of full decay varies from one area to another, and the people of each country are more aware of that. As noticed from the above question, the bones of the dead are existent and haven`t reached full decay.
Therefore, it isn`t permissible to take the corpses out of these Fosqiahs and exhume the graves therein so as not to violate the sanctity of the dead. And Allah knows best.
