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Abstract Extended reach in coiled tubing (CT) interventions and drilling (CTD) is essential for accessing complex reservoirs and improving operational efficiency. While previous studies have examined lateral path tortuosity, the impact of the buildup section trajectory on CT reach remains largely unexplored. This paper quantifies how buildup trajectory influences extended reach and proposes optimal well trajectory to maximize lateral access. Using industry-standard Tubing Force Models (TFM) combined with buckling and tubing yield criteria, we first analyzed the buildup section under the assumption of constant dogleg severity. Our findings reveal that buckling can occur only at the start or near the end of the buildup section—never in the middle—providing practical guidance for trajectory design. We identify an optimal constant dogleg severity, typically 1.5° to 2.0° per 100 ft, that maximizes lateral reach; dogleg values above this optimal reduce reach following a simple scaling relation. Relaxing the constant dogleg constraint, we derive the optimal buildup trajectory shape, characterized by lower dogleg severity at both ends. Field data comparisons corroborate our predictions, providing practical validation. This study delivers comprehensive insights and guidelines for operators to enhance CT extended reach by optimizing buildup section design, supported by theoretical analysis and field validation.