Renewable sources come with much more uncertainty. Fossil fuels created backbones of predictable flows under a relatively stable demand; however, tomorrow's energy system requires not only expansion, but anticipation. This shift forces a rethinking of what resilience truly means, and a viable path forward is to focus on flexibility, a spatially explicit design challenge in both principle and practice. This master's thesis, developed as a joint program between TU Delft and Stedin, takes the practical route. It approaches the challenge through a large and detailed application where flexibility is mapped across 283 high voltage substations and 359 transmission lines with projections to 2050. Decisions taken today will shape the reliability, security, and economic viability of tomorrow’s power system. In the Netherlands, the II3050 report offers high level guidelines for grid development while it also makes clear the need for more operational detail: flexibility must be placed and sized with greater precision. And yet, that scale remains undefined. This thesis addresses that gap. Using the open-source modelling framework Calliope, it builds a national model of the Dutch high voltage grid. It reconstructs its fully mapped topology and uses it as the structural foundation. From there, it builds a model to identify where and how much flexibility matters the most, resulting in cost-optimal and near-optimal portfolios of design alternatives. The goal is not only to chase the optimal outcome, but to identify solutions that consistently perform well. Modelling to generate alternatives (MGA) techniques are applied to the case through the SPORES method, which improves by also accounting for the spatially distinctive options rather than just costs. Supporting tools have been built to assess where technologies and locations recur most frequently across optimal portfolios and different scenarios. No-regrets analysis has been carefully performed to find those robust choices that hold their value even under varying system conditions.