Illegal construction is constructions without a valid / right construction permit. Besides the potential technical hazards on uncontrolled construction sites and in finished buildings, illegal building activity can be a major environmental violation when the works encroach upon preserve areas like nature reserves.
First, RTI can be used to pressurize Government Officials to act against builder. If Government Officials is pressurized take action against the builder, the builder will be forced.
We can go to the municipal corporation and file a complaint against unauthorised construction. Also file a petition in the High Court under Article 226 of the Constitution of India.