If you are not able to construct the house within 3 years and transfer the plot then in that case if you are selling property after 2 years of purchase then it will be short term capital gain otherwise it will be long term capital gain and in that case if the cost of new asset was more than capital gain then the cost of new asset for calculating capital gain would be cost less capital gain claimed as exemption and then the capital gain will be calculated.
In second scenario there is use of transferring the land to wife as you will be considered the deemed owner but if you transfer it to your mother as a gift then her cost would be the cost to previous owner as per section 49(1) as in her case also capital gain will be calculated as mentioned above. The only benefit you can get here is the slab benefit available with your mother. So both the case are same and if you are going to invest in new house may be you can claim exemption u/s 54F as now you are transferring land and in that case you need to invest entire sale consideration and not only the capital gain to claim exemption.
It will attract litigation so just be careful and plan everything properly.
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