The soil-structure interface, representing the contact surface between soil and structures such as buildings or rocks, assumes critical importance in water-related projects like earth dams, cut-off walls, side slopes, foundation pits, and water tunnels. Monitoring reports and studies consistently identify this interface as a vulnerable zone susceptible to seepage-related accidents. Despite this recognition, research on the topic has been limited, with a predominant focus on experimental methods that tend to underestimate permeability. This paper presents an analytical model, grounded in capillary theory, for calculating the permeability coefficient of the soil-structure interface. The study explores the differences in permeability between the interfacial soil and far-field soil. The study concludes that the higher porosity of the interfacial soil (porosity ≥ 0.48) compared to the far-field soil (porosity: 0.26 ∼ 0.48) is a key factor in rendering the soil-structure interface susceptible to seepage. The calculated permeability coefficient of the interface, in relation to experimental values, exhibits a consistent ratio of 2.5 ∼ 2.6 when the soil porosity is below 0.43, indicating reliable predictive utility. Moreover, the permeability coefficient of the interfacial soil proves to be at least 4.05 ∼ 6.67 times larger than that of the far-field soil, potentially leading to the creation of preferential seepage channels at the interface.