Getting a house to appraise with a basement

[Michael McDonald]

I am trying to sell my home in the Greenville South Carolina area that has a basement.  When the house went on the market i got 3 offers above my asking price in less than 24 hours.  The highest one coming in $20,000 over the asking price.  We did all the inspections and everything has checked out well.  The problem we are having is almost half of the finished house square footage is below grade which is a walkout basement.  The appraisal came in 50k below the contract.  Any recommendations