Recently, the number of unsold housing units in South Korea has begun to rise again. This study examines the factors influencing the unsold housing rate using municipality-level panel data from 2015 to 2023. It specifically focuses on the effects of various housing supply types, including urban development, redevelopment, and reconstruction projects. Empirical analysis using a dynamic panel model shows that household growth reduces unsold housing rates across all regions, while a higher share of newly constructed housing significantly increases unsold housing rates in non-metropolitan areas, indicating a greater sensitivity to supply concentration outside the capital region. More interestingly, an increase in urban development projects is found to raise unsold housing rates, whereas redevelopment and reconstruction projects help reduce them. Housing price variables also exhibit regionally divergent effects: in the metropolitan area, both sales and jeonse price indices are significant, while in non-metropolitan regions, sales price growth stimulates investment demand and reduces unsold housing rates. Based on the empirical results, this study enhances understanding of the structural causes of unsold housing by examining various forms of new housing supply. The findings underscore the need for a strategic shift from expanding undeveloped land toward utilizing existing urban areas to create a sustainable housing supply system amid population decline and regional stagnation.