## 2 Theoretical Background  
### Preliminary Design and Conceptual Cost Estimation in Architectural Planning  
The preliminary design of a construction project refers to the phase in which a preliminary investigation is performed, and a master plan is established according to the basic conception of the project proposed by the owner agency; the conceptual design is performed based on the investigation and master plan [9]. In South Korea, the work in the planning stage is divided and categorized according to the type and scale of the project; the owner, designer, and construction manager participating in the work analyze and plan requirements for the project, review its economic feasibility, and perform preliminary work for a more detailed development of the building design, such as the generation of building floor plans and building layouts, as well as facilities planning of the building. Among the planning work performed in the early phase of the project, matters related to construction cost planning are considered important [10]. These issues are critical because the decisions made in the early stage of the project, including those related to construction cost planning, have greater effects than those made in the later stages of the project. In addition, the content and nature of construction cost planning and estimation vary according to the progress of the project, thus requiring more detailed construction cost estimation [11]. However, to perform construction cost planning in the architectural planning stage when the details of projects have not been confirmed in many aspects, it is necessary to secure the available information at the planning stage and to reflect the information accordingly. With project progress, the available information becomes more detailed as the developed design is highly detailed; nonetheless, the amount of information available in the preliminary and schematic design stages in the early phase of the project is considerably less than that available in the phases with detailed design [3]. However, as can be seen in Figure 2, the project information that is applied to decision-making is limited, and thus the planned expenditure in the early phase of a construction project is relatively small compared with that in the later phases, but the decisions made in early phase of the project have considerable effects on the expenditure in the later stages of the project [4]. Therefore, construction cost planning in the architectural planning stage supports the decision-making of the project owner, facilitating project execution and enabling the determination of the direction of project implementation [12]. Figure 2: Level of influence and information ([PERSON] 1976). The construction cost estimation process described above is referred to as \"conceptual cost estimation,\" although the definition of the term differs between institutions and experts. In addition, regarding the timing of conceptual cost estimation by project phase, the names used in the existing literature or previous studies are slightly different, but the applicable phase is from the preliminary or planning stage to the completion of the design development or the middle of the construction documentation phase [7]. That is, the previous studies are similar in that the cost and budget have been estimated with limited information in the early phases of construction projects, and the estimates have been utilized to evaluate the economic feasibility and validity of the projects. However, owing to the nature of construction projects, most budget estimation is performed in the planning stage, and economic feasibility and validity are analyzed based on these estimates; therefore, it is considered that conceptual cost estimation should start from the planning stage [6].