Final answer:
The income approach is the most credible method for appraising an older apartment building without recent sales data of similar properties, focusing on potential income generation. The cost approach could also be a useful method, but the sales comparison approach is less credible in this scenario without recent comparable sales data.
Step-by-step explanation:
The most credible approach for appraising an older apartment building in an established neighborhood, especially when there are no recent sales data of similar properties, would be the income approach. This valuation method focuses on the potential income that the property could generate, which is particularly relevant when direct market comparisons are not available. The income method involves estimating the property's future income streams and capitalizing these into a present value. Considering factors such as rent levels, occupancy rates, operating expenses, and appropriate capitalization rates, the appraiser can arrive at a value even with the absence of directly comparable sales data.
Alternatively, the cost approach could also be considered credible; it estimates what it would cost to replace or reproduce the existing structure, less depreciation. However, the sales comparison approach would be less credible without recent comparable sales, and the comparative unit method is not standard terminology in appraisal and may refer to an element of the sales comparison or income approaches.