Capital gain confusion

1. Indexed cost of half of the plot of land will be 1.5 lacs × 785 ÷ 426 = 276408.(Assumind plot of land was divided exactly half)

CG for AY 2012-13 will be 15 lacs - 276408 = 1223592 (Presuming claimed exemption under 54F by fulfilling all the conditions of it. Hence entire CG exempt then. However, if not claimed any exemption at that time, CG will be LTCG and will be taxed @ 20% of CG)

2. Now coming to AY 2014-15, individual valuations of plot of land and house property is required. If it was a joint sale deed and not separated then get separate valuations done by asking the Registrar, value of the land and house at the time of sale. Individual valuations are necessary to ascertain the CG individually.

Now the indexed cost of land will be 1.5 lacs × 939 ÷ 426 = 330634 and cost of house will be 15 lacs. (since entire amt was invested in construction of house as mentioned in your statement. If not then exemption calculations done previously will change accordingly) Indexation will not be applied to house as it is a short term CG since it is sold within 3 years of purchase. 

After getting individual valuations on the date of sale., CG will be Sale - Indexed cost / Cost as the case may be.

Important to note that if exemption u/s 54F is claimed earlier then entire entire exemption would be disallowed now and the same will be taxed as LTCG now. i.e. CG of AY 2012-13 of 1223592 will entirely be taxed as LTCG.

House will be taxed as STCG and Plot of land sold in AY 2014-15 will be taxed as LTCG.

No he cannot avail the exemption back because it is clearly stated in sec 54F for not purchasing or constructing the house within period of 1 year/3 years as the case may be for purchase or sale from transfer of original asset