Capital gain

NHAI has Complusory Accquired land and Building of one client and TDS has been Deducted Under section 194 LA, as i had come to conculsion since land acquired is rural agriculture land and which not capital asset as per section 2 (14)  hence no capital gain. But My Query about actual cost of building since client does'not have any bill for construction nor any purchase deed nor the idea of the year of construction .

How to determine actual cost of building so to compute capital gain ?