The feasible operation region (FOR) allows capturing the aggregated flexibility potential of DER within radial distribution networks, while respecting the technical restrictions of both plants and grid. This thesis proposes a novel approach to compute the FOR, the Linear Flexibility Aggregation (LFA) method, based on the solution of a sequence of linear OPF. With the objective of reducing the computation time, without compromising the accuracy of the assessed FOR. It is shown that the proposed method provides a considerable reduction in processing time compared to similar methods, e.g. Monte-Carlo simulations or non-linear OPF-based methods.