Faulty Row House Construction By Bulider

Respected Sir, I want urgent legal advice on subject as mentioned above. My Parents have purchased a row house (4BHK) from a Builder worth rs 40,00,000/- ( possession was given to us in march 2014. At the time of possession, the Quality of construction was appearing ok so parents had signed the papers(stamp paper) regarding possession being given by the builder in proper conditions. But after the rainy season has started actual quality of construction has started appearing. Cracks have appeared on the walls(from outside) especially at the column faces, paint is cracked, roof has started leaking owing to faulty slope being provided. Entire Rooms were Filled with water due to water leakage. We had Talked to Builder But no positive response. Seeing the present condition of house which has just appeared within 6 months of possession being provided to us, it is estimated that within 2 years this house will not be in the situation to live in. So Sir please some advice that may help to get legal remedy in this case.