The county replaced its old office building with a new structure. Rather than destroy the old office

Question:

The county replaced its old office building with a new structure. Rather than destroy the old office building, the county decided to convert the old building and use it as a storage facility. Why would the old office building need to be evaluated for impairment? If it was decided the building was impaired, what would be the most appropriate method for estimating the degree of impairment?