Cost basis would be the amount you originally paid for the property plus any improvements made over time. Fair Market Before could come from an appraisal you had made during the 12 months before the casualty or an estimate you could request from a local realtor or appraiser. If you live in a Federally declared disaster zone, you could use the cost of repairs to determine the fair Market After by subtracting the repair amount from the "Before" value. You can only do this, though, if you already had the repairs done and paid for. Otherwise, you would have to obtain the estimate of Fair Market Value after from a realtor or appraiser.
