Point to be considered here are
First, does an assessee acquire an asset by booking a flat Capital asset has been defined to mean property of any kind. Hence, a right to obtain conveyance of an immovable property can be construed as a property. Accordingly, if the booking agreement and allotment terms of the builder give a right to obtain conveyance on the flat, that itself will be an asset under the Income Tax Act.
Second, whether the gain on transfer of such rights in the flat or on transfer of the flats after talking possession thereof is short term or long term The deciding factor is the date of acquisition. Indian tax authorities are giving priority to the facts of the case. It has been held in various rulings that the crucial date is the date of allotment of the residential flat and the payment of instalment is merely a follow-up action. Hence, the date of issue of an allotment letter / agreement to sell can be construed as the date on which the assessee has obtained the conveyance on the flat.
so depending on the factor on which year you have obtained conveyance on the flat, that year should be considered as cost of acquisition of the flat for the purpose of the capital gain.