Deductions & credits

We have a similar situation.  We've seen the allocation method in the code.  (Purchase price divided by acres = cost basis by acre)  where is the reference to using an outside appraisal to affect cost basis?  ( we bought 48 acres and subdivided into four 2.5 acre lots, consuming 10 which we'll sell).  We think the remaining 38 acres have lower value than the 10 we sold off)

 

Thanks,

 

Don