There are three main forms of depreciation. They are physical deterioration, functional obsolescence and external obsolescence. There are ways that both can actually be extracted from the market, but most appraisers use age estimates and percentages based on their observation of the building and their knowledge of other similar buildings in the market.
External obsolescence is a reduction in value from forces outside the property itself.
If a nuclear power plant is constructed next to a school, that may cause external obsolescence to be created if the value of the school is adversely effected. There are whole courses on estimating depreciation. I hope this thumb nail helps. The only people who can render an opinion of that are realtors or appraisers in your market. All Rights Reserved.