The purpose of the this research is to develop the cost planning system to be used step by step during the production process on construction projects of public offices. The purposes of the research are as follows. 1)System development for cost planning to achieve business decisions. 2)Improvement for traditional cost planning system by public clients. 3)System development for change order and Value Engineering clarified predictable construction costs. 4)System development to reduce workloads for estimating construction costs.