Best way to avoid long term capital gain tax

My Uncle sold a Plot  at  7 L  

The thing is that plot he got againest 1994 around 15K from  City Development 

city development issued plot againest a House My uncle own which is  half construction and city development office took over that plot and issued another plot later in 2005         

Now i Know that  15K was price ucle paid and got recipts  and can add inflation index  2 

But I Want to know that a half construction house which city development took   can  My uncle Add them into cost of that plot  cuz uncle dont have any Reciepts or proof for house which city devlepement took  except its in the file of city development office   also how to justify  of cost of that half construction house  ?    which city devleopment took in 1994