成本法為不動產估價主要方法之一，結果的正確性端賴土地價格、重置（重建）成本以及建物折舊的合理估算。台灣對於成本法研究並不算多，尤其缺乏對於建物折舊的實證討論。因此對於折舊的估算大多仰賴經驗法則，或是政府公佈之相關數據。這些經驗或官方數據，仍然缺乏較科學的檢驗。利用台北市近年公寓住宅交易資料，本文推估建物之市場折舊，也就是屋齡對於建物價值的影響。為避免不動產價格的空間自相關問題，本文採用空間遞延以及空間誤差模型。實證結果顯示公寓建物折舊呈現凸型路徑；建物興建完成初期折舊較後期為快，而非經常假設的固定額度折舊。此外，區位條件較佳地區的建物，折舊速度較區位較差地區的建物緩慢。這種建物折舊的空間差異，應該是地區間外部性退化速度不同所導致。此外，現行法規之折舊率明顯低於本文推估之市場折舊率。在房屋標準單價正確的前提下，低於市場水準的法規折舊率將造成房屋現值（房屋稅稅基）明顯高估。就屋齡41年的公寓來說，折現後的法定稅基高出市場推估稅基約1.7至2.2倍。

The cost approach is one of the primary approaches to valuing real property, and the accuracy of which depends on satisfactory estimates of land value, replacement or reproduction costs and building depreciation. The lack of scientific data on building depreciation in Taiwan has led to heavy reliance on rule-of-thumb. This investigation, using data on recent transacted condominiums in Taipei City, finds that depreciation follows a convex path. Condominium building value decreases faster during the early part of the life of a building than during the late part. Furthermore, depreciation rate varies among condominiums in different neighborhoods. Condominiums in neighborhoods with superior amenities depreciate slower than those in neighborhoods with inferior amenities. Additionally, the market-indicated depreciation rate significantly exceeds the legally-specified rate. Assuming the replacement cost is correct, the housing tax base is over-estimated. For example, the accumulated tax base for a condominium aged 41 years old is over-assessed by 0.7 to 1.2 times.