The cost approach was historically prepared as a part of most commercial real estate appraisals. However, the compunction to include the cost approach (when it was not relevant) has dissipated over the last 20 years.
The principle of substitution is the technical basis for employing the cost approach. According to the principal of substitution, a prudent buyer would not pay more than the cost to build a like property. In other words one would not spend $2,000,000 to purchase a new apartment complex if they could build it for 1,500,000.
The cost approach value is the sum of the market value of the land, depreciated replacement cost and entrepreneurial effort. Land is typically Valued using the sales comparison approach. The replacement cost is the cost to build a building of the same quality and functional utility as the subject property. (Reproduction cost is the cost to build an exact duplicate. This approach is used occasionally for old buildings built using materials and or types of craftsmanship not currently used.)
External obsolescence occurs when circumstances outside the subject property's boundaries negatively impact its value. For example, an office building in New York would suffer from external obsolescence if Manhattan office occupancy fell from 93% to 75%. A mansion built next to a slaughter-house is another example of external obsolescence.