What is the most reliable source of information in the cost approach?

In the cost approach to property valuation, the most reliable source of information is indeed the taxpayer's accounting records. These records provide detailed and accurate data regarding the actual costs incurred by the taxpayer in acquiring, upgrading, or maintaining their property. This firsthand information reflects the unique characteristics and specific uses of the property, which can translate into a more precise estimation of its value.

Taxpayer accounting records can include asset listings, depreciation schedules, and detailed financial statements, which all contribute to a thorough understanding of the property's value from a cost perspective. They provide insights into actual expenditures on materials, labor, and any enhancements made to the property over time.

While market reports, government regulations, and estimator guidelines can provide useful context and supplementary information, they do not typically offer the same level of specificity and accuracy as a taxpayer's own accounting records. Market reports reflect general trends and prices that may not pertain to a specific property, government regulations can dictate what should be considered but not the actual costs involved, and estimator guidelines are useful for outlining typical scenarios but may not reflect the unique factors relating to an individual property’s cost. Therefore, taxpayer's accounting records stand out as the most reliable source in the cost approach to valuation.