No, you cannot claim 100% property value as cost base for capital gains. Your actual cost of acquisition is the total amount you paid (50% to assignor + 50% to developer + incidental costs like stamp duty/registration).
Cost of acquisition (Sec 55): Full purchase price/expenses to acquire the asset, regardless of sale deed value (which was only for 50% as assignee).
Deed shows 50% because assignor transferred their rights; you paid remaining directly to developer total outlay = your COA.
TDS/stamp on 50% irrelevant; COA is your expenditure, not deed figure. Courts uphold actual payments.
? Keep bank statements/digital trails for both payments. Indexed COA = total cost x CII ratio. Gain = Sale price - Indexed COA.
Claim full actual cost in ITR. AO may accept with docs; low dispute risk.
Consult CA.