This study examines the effect of legal subject and legal object complexity on legal-document completion duration and its relationship with opportunity cost in built-to-suit (BTS) tower projects at Protelindo Group. Legal-subject categories comprise individuals, heirs, legal entities, government institutions, and religious-property managers, while legal-object status distinguishes certified and non-certified land. A quantitative explanatory design was applied using a five-point Likert questionnaire administered to 200 respondents from the Protelindo Legal team, SITAC BTS team, and experienced turnkey vendors. Data were analyzed using Partial Least Squares Structural Equation Modeling (PLS-SEM). The results show that legal subject and object complexity has a positive and significant effect on completion duration (β = 0.758; t = 24.588; p < 0.001), while completion duration positively and significantly affects opportunity cost (β = 0.325; t = 5.403; p < 0.001). Legal complexity also has a direct positive and significant effect on opportunity cost (β = 0.543; t = 9.470; p < 0.001). The model explains 57.4% of the variance in completion duration and 66.8% of the variance in opportunity cost. The findings indicate that legal complexity should be screened at the front end of BTS land-acquisition processes because incomplete documentation, repeated clarification, external coordination, and administrative rework can extend lead time and consume productive resources.