A narrow backfill zone is formed when retaining walls are built near existing stabilized structures (e.g., rock faces). In such circumstances, the classical passive earth pressure coefficient is no longer applicable, and a correction factor is required for the design. This paper aims to develop analytical solutions for estimating the passive earth pressure problem of narrow cohesive-frictional backfills behind retaining walls. The novel arched differential element method considers both effects of the horizontal shear stress in backfills and the soil arching, and it is employed to estimate the passive earth pressure distribution along with wall depth. The solutions are compared against those published experimental data, analytical approaches, and finite-element limit analysis solutions. The factors influencing the distribution of passive earth pressure are also undertaken using a series of parametric studies. To implement the derived solutions into a routine design, a modified practical design equation is presented following the standard Coulomb's solutions. This work provides a theoretical guideline for the initial design of retaining walls with narrow soils, and it should be of great interest to practitioners. @en