Sayyed Mohammad Al-Musawi,
Sayyed Mohammad al-Musawi is originally from Iraq and heads up the World Ahlul Bayt Islamic League in London. Other than being involved in various humanitarian projects, he frequently responds to religious questions. In the past, he has also spent significant time in India guiding the community.
Even if the heirs allow, it is not permissible to dig an existing grave to bury another dead body if the previous dead body still exists in the grave. If the previous dead body has completely decayed and became soil, then it will be permissible to bury another dead body in the grave which has no remaining. In such case, if the land of the grave was purchased by the heirs, their permission is must to bury another dead body in that empty grave.