Getting an appraisal months later, when the taxpayer no longer owns the house, is not only impractical, it's meaningless.  And it's the sale of one property, but with two holding periods.  

Hypothetical:  Taxpayer owns Lot A for 20 years.  He buys adjoining Lot B and then sells them both in a single transaction six months later.  Since the combined property was created less than a year ago, it's all short-term?  I don't think so. 

There are some interesting rules that apply only to low-income housing properties that deal with breaking down the holding period based on when parts were refurbished or rebuilt.  Those don't apply here, but they show recognition of the question.