This paper presents an overview of a computer-based site layout model and focuses primarily on the project setup phase. The developed model has four modules: user interface, database, project module, and layout module. Setting up the project in the proposed model is carried out by the project module, utilizing open architecture concept. The main advantage in the open architecture is to allow for the incorporation of userdefined objects if they are not readily available in the model. The objects required to define a site layout problem are clustered into three tires: 1) Site Objects, 2) Construction Objects, and 3) Constraint Objects. The model is implemented in a CAD environment using an object-based approach. The structure of each of the three tires is described, and the mechanism of object selection/creation for a site layout project is explained. The paper describes the components required to implement an open architecture for site layout object selection along with their respective environments. The developed model can be easily extended to similar applications such as floor plan design.