I have a Doubt that If Pakistan can Build Dams on P-O-K then why India Can built it in J&K
The River Waters flow from India to Pakistan.
As such to be fair India is not allowed to build Storage Dams but can build Dams for Power Generation so that the total amount of Water to Pakistan is ensured. This condition is fulfilled by India while constructing the Dams in the State of Jammu & Kashmir. India is however allowed to take waters from the Rivers allocated to Pakistan amounting to what is and has been already used by the People of Jammu & Kashmir by the "Loval" people living in the areas through which the "Rivers allocatted to Pakistan" flow.
The only hindrance to Pakistan In building Dams in Pakistani Occupied Kashmir is that India still claims that part of the State of Jammu & Kashmir as its own and thus does bring up the matter of protesting against Pakistanis building dams in Pakistani Occupied Kashmir.
One cannot make any statement of the Legality or Practicality of such a claim.