Is it permissible to build a "fusqiyah" (a small basin-like structure often used for irrigation or ritual washing) for burial above a lahd (a niche in the side of a grave) that contains deceased members of the same family, knowing that there will be absolutely no disturbance to those inside the lahd?

The mentioned issue does not fall outside of two scenarios:

Scenario One: Exhuming the grave and then building a فسقية (cistern/structure) over it and burying other deceased individuals within it. It is permissible to exhume obliterated graves where it is highly probable that no trace of the deceased remains. If their body and bones have disintegrated and turned into dust, then it is permissible to exhume it and bury someone else in it. It is not permissible to build a فسقية with bricks and plaster except out of necessity (such as fear for the deceased from wild animals or the collapse of the grave due to a flood). The height is determined by the necessity, and it is not permissible to raise the grave above the ground level by more than a handspan.

Scenario Two: Building a فسقية over the grave without exhuming it and burying deceased individuals on top of other deceased individuals. It is not permissible to bury deceased individuals on top of other deceased individuals except out of necessity, due to the prohibition against building upon a grave, adding to it, or plastering it, and because it involves stepping on the graves of the deceased, which is forbidden. If necessity calls for stepping on the grave, then it is permissible. If there is a strong presumption that no trace of the deceased remains, then exhumation is preferable; otherwise, they should be buried on top of them, and stepping on the grave should be avoided as much as possible. It is not permissible to bury in this graveyard if there is no necessity for it.