Fair market value
(FMV) is the price at which the property would be sold between a willing buyer and a willing seller, each having knowledge of the relevant facts. The difference between the FMV immediately before the casualty or theft and the FMV immediately after represents the decrease in FMV because of the casualty or theft.
The FMV of property after a theft is zero if the property is not recovered.
FMV is generally determined by a competent appraisal. The appraiser's knowledge of sales of comparable property about the same time as the casualty or theft, knowledge of your property before and after the occurrence, and the methods of determining FMV are important elements in proving your loss.
The appraised value of property immediately after the casualty must be adjusted (increased) for the effects of any general market decline that may occur at the same time as the casualty or theft. For example, the value of all nearby property may become depressed because it is in an area where such occurrences are commonplace. This general decline in market value is not part of the property's decrease in FMV as a result the casualty or theft.
Replacement cost or the cost of repairs is not necessarily FMV. However, you may be able to use the cost of repairs to the damaged property as evidence of loss in value if:
The repairs are necessary to restore the property to the condition it was in immediately before the casualty;
The amount spent for repairs is not excessive;
The repairs only correct the damage caused by the casualty; and
The value of the property after the repairs is not, as a result of the repairs, more than the value of the property immediately before the casualty.
To figure a casualty loss to real estate not used in a trade, business, or for income-producing purposes, measure the decrease in value of the property as a whole. All improvements, such as buildings, trees, and shrubs, are considered together as one item. Figure the loss separately for other items. For example, figure the loss separately for each piece of furniture.
Form 4684 instructions