Which of the following categories of adjustment is NOT essential when preparing a market data estimate of value?

When preparing a market data estimate of value, the category of adjustment that is not essential is the original cost of the building. This is because the original cost does not necessarily reflect the current market value or demand for the property.

In real estate, market value is determined by what buyers are willing to pay for a property based on comparable sales and current market conditions. Factors such as market trends, comparative sales, and location characteristics are critical since they directly influence the valuation. Market trends provide insights into whether property values are rising or falling, comparative sales offer a benchmark to gauge how similar properties are valued, and location characteristics affect desirability and thus impact value.

Therefore, while the original cost may have significance for accounting or investment return assessments, it does not play a critical role in the more immediate and situational analysis required for determining a property's market value.