This research proposes a significant reduction in the processing time to solve the concurrent AC multistage transmission network expansion and reactive power planning problem with security constraints, by an innovative search space reduction strategy. Initially, the concurrent planning problem is modeled as a mixed-integer linear programming (MILP) problem, using an AC branch flow formulation to represent the steady-state operation of the transmission network. The innovative strategy consists of obtaining a stage-by-stage solution pool of the MILP model as a static problem, to identify the significant candidate lines. Therefore, those lines considered insignificant would not be considered as candidates in the multistage problem. Then, with the updated database, it is possible to solve the multistage MILP problem with a reduced search space. The evaluation of the proposed methodology is done using the IEEE 24- and 118-bus test systems, showing the performance of the proposed methodology.