Decision-making in green design is a very complex process as it contains several alternatives for various criteria. In order to overcome this complexity, a holistic approach for the decision making process is needed. However, when green design is discussed, the main concerns of researchers or decision makers are reducing energy consumption. In fact, user involvement in sustainable decision making process is also a critical factor for the project success. Unfortunately, user centered design approach to green existing buildings especially the educational ones are scarce in the research area. Therefore, this thesis aims to develop a decision making model through a user centered design approach for Bilkent University Library. To achieve this, firstly user needs were identified and then they were prioritized via Analytical Hierarchy Process (AHP) by the designers. As result, acoustic comfort had the most importance weightings among the other design factors. Additionally, the needs of the different user groups of the library building, the students and the staff, compared, and it was observed that the library staff need the green solutions more than the students do.