Cost estimation is one of the most important tasks for succeeding projects. Recently, as the construction environment is changing more rapidly, it is getting more difficult to estimate the project’s cost accurately. Therefore, for an effective decision making, it is necessary to develop a sustainable cost estimating model which is able to deal with various changes. In this research, in order to develop the model, four conditions, reliability, efficiency, expansion and flexibility, are extracted. Then, Case-Based Reasoning is applied, and this research proposes cost database, influence factor and an estimating method which are three key elements for each task. As an example, reinforced concrete work of residential projects is used. Lastly, analysis of reliability is carried out for the estimating model. Through this research, it is expect that many studies on a sustainable estimating model is encouraged continuously in the future.