Obtaining Occupancy Certificate and Completion certificate can be mandatory under respective State laws. As per these state laws, one cannot legally move into a building unless the developer gets an occupancy certificate from the respective local administrative or civic bodies. The corporation or municipality can ask the apartment owners to leave such illegally occupied flats or impose heavy penalties in the name of ‘regularization’.
No doubt the builder is yet to provide you the CC which is required for legally occupying the property purchased but since you have taken over possession, it wold be better you occupy the same without CC and demand the same to the builder by communicating your requirement in writing under acknowledgment to the builder an secure this as evidence for future litigation purpose.
The legal possession of the Flat by the Owner is valid only with the Occupancy Certificate.
In many of the cases, the building plans of these apartments are approved but they do not get OCs due to not sticking to the approved building plan or due to other violations.
The builder has to obtain OC. However, the Flat owner can also apply for OC. If you have already occupied your FLAT without OC, you can approach the Local Corporation or municipality and apply for OC. If the building has been constructed as per the approved plans, building standards, fire safety standards and if it also has BCC then you can get OC in 30 days (legally). However, the law says that if the building has been constructed without any plans it cannot be regularized (if there are structural deviations) and OC may not be issued in such cases.