Question about capital gain

In case of property received by succession after 1st April 1981 which is originally acquired by the previous owner before 1st April 1981 the assessee has received three floored house. He intends to sell first and second floor and retain ground floor. How will the cost of acquisition be arrived in case of two top floors where no separate figures are available for construction of house. In this case cost of land acquired is availabe separately.