工期展延為工程爭議中最常發生之類型，其發生常因契約明定，不論工期展延事由為何，皆不補償承包商因工期展延造成之費用損失；事實上，此項約定應適用在工程展延原因系不可歸責於雙方時，方屬合理；基於民法精神，雙方之責任應予以明確規定，若是業主所導致工期展延，則業主須就相關之管理費用給予補償；但契約中或許未明訂補償方法，或是以不公平之方式約定，皆使得管理費用之計算成為契約的漏洞。 在未約定展延工期的計算方式下，承包商對於工期展延之管理費提出求償，實務上通常以「比例法」與「實支單據法」來計算，但即使承包商舉證後仍無法證明該單據為可歸責他方所衍生之管理費，使承包商透過爭議處理程序索賠，管理費用之求償金額比例較其他爭議類型低。 本研究希望藉由作業基礎成本制（Activity Based Costing，以下簡稱ABC）之架構，找出「實支單據法」在計算過程中合理的證明流程，並針對無法確實證明的項目採用「比例法」配合適當的歸屬方式，建立施工管理費用之計算模型，於日後可供業界使用計算出更為合理的求償金額，也可藉由ABC模型的建立，使營建業能確實計算出管理費用對專案造成的影響，進一步正視對管理費的計算控制。 Among the variety of construction disputes, extension of time (EOT) is the most prevailing type. When the cause of EOT is found attributable to the project owner, a derived cause of action may be justified, namely the reimbursement of management fees arising out of the extended time in the project schedule. Naturally this derived form of dispute is a major source of quarrel between the parties. The heart of it lies not in whether such management fees were actually incurred, but in how much of the fees sought by the contractor may be appropriately justified. In literature, there are a substantial number of methods, which proposed by pundits who specialize in resolving construction disputes. The genius in developing methods to calculate the management fees upon EOT also reflects the fact that the potential for parties to disagree on how such fees may be calculated is great. After examining quite a number of cases in mediation, arbitration and litigation, it is found that the core of the problem is an evidentiary one. However, the contractor’s claim is rejected by the adjudicator, because he can hardly prove that the costs incurred are linked to the EOT and the sum proposed is supported by verifiable causes. This work is a direct response the filling the evidentiary gap mentioned herein. The proposed solution is to employ the “Activity Based Costing” (ABC) approach which has been quite well accepted and dominant in the manufacturing sector. The fundamental idea of ABC is to adopt a set of driving factors which would link any cost item to the respective cause of construction activity. By using the ABC framework in daily site management, the actual cause-and-effect relationship about the details of site management fees is enforced. The result is hard-earned transparency of actual costs incurred by any owner-caused EOT, thus removing any vagueness for justifying the reimbursement. An additional aim of the work is to propose a guideline for streamlining cost management at the site level.