Most of the PPP (Public-Private Partnership in Infrastructure) projects in Seoul are large-scale such as urban railroads, roads, tunnels, parking lots, and cultural facilities, and are profitable BTO (Build-Transfer-Operate) with high demand for use. Since such large-scale projects are difficult to promote with private funds only, government financial support is needed.

Government support for PPP projects varies from project to project, but there are construction subsidies, toll differences exceeding public charges, and minimum operating expenses. Since these supports are continuously provided during the construction and operation period, efforts to reduce the government's financial burden are required. Thus, as a path towards reducing the financial burden of PPP projects in Seoul, it is necessary to study the types of supplementary projects that reduce the financial burden, by promoting geographical supplementary projects close to the main project.

Since the private sector has limitations in improving the profitability of the main project only with the income of user fees for PPP projects, mechanisms are needed to further increase profitability by using supplementary projects separately from the main project. These supplementary projects may only be conducted for the projects listed in the PPP Act. Additionally, supplementary projects are promoted as separate projects from the main project.

The purpose of this study was to revitalize the promotion of supplementary projects in the Seoul PPP project. To this end, the types of supplementary projects suitable for Seoul are presented, by analyzing the construction conditions such as the infrastructure and land status of Seoul.

To present the project types of Seoul's supplementary projects, the results were analyzed as follow through surveys and expert advice.

First, the difference in perception and yield of beneficiary burden, profitability, project benefit, and efficiency were analyzed by comparing the supplementary project with the PPP main project.

Second, from the survey, supplementary projects that the private sector intended to promote in Seoul were investigated, and the types of supplementary projects that could be promoted were classified by reviewing related laws and regulations for each project.

Third, the types of supplementary projects were divided into new site use projects, complex projects in existing facilities, and system construction projects that do not require land. Additionally, related laws were reviewed to ensure that these projects were executed well.

Additionally, related laws and regulations were reviewed by comparing them with the PPP Act, so that these projects could be executed well.

Based on this, this study classified and presented the types of supplementary projects that can be promoted in Seoul, and explains that to revitalize supplementary projects, it is necessary to improve the system such as the PPP Act and the Program for PPP.