This study presents an advanced methodology for stiffened plates subjected to combined in-plane longitudinal compression and lateral loads. The proposed methodology is based on comprehensive numerical parametric analyses, and it utilises newly developed design curves for the lateral pressure limit and tailored empirical formulae for stiffened plates to increase the precision of ship structural designs. Plate–stiffener combination (PSC) members are used for the limit state analysis, which introduces various positions and initial deflection shapes specific to the PSC models. Local buckling-shaped deflections of the stiffener web are also incorporated into the analysis, which closely mirrors the real-world welding conditions of stiffened plates and provides deeper insight into their load-bearing capacities. These findings highlight the importance of accurately selecting the positions and initial deflections of stiffeners in PSC-based analyses to ensure that structural predictions are safe and reliable. The proposed method is grounded in conservative PSC models and represents a significant advancement in ship structural design in terms of safety and practicality.