The inclusion of distributed energy resources and electrification of heat and transport is creating additional supply and demand stresses that distribution grids were not originally designed for. The flows of energy and revenue are changing in magnitude and direction, making these grids more dynamic over time. In this changing landscape, the traditional approach for planning in distribution networks of "oversize, fit and forget" is not enough. A review of the literature in grid planning shows that the current focus is on transmission-network-inspired methods that are not realistically scalable for the distribution network case. Accordingly, this thesis presents a collection of novel technical and economic methodologies to transform the planning paradigm into an active one, including sharing economy concepts. First, this work presents a technology-agnostic impartial method to assign customers with export capacity for distributed generation. Subsequently, a new method to determine location, size, and prioritisation of flexibility resources at the distribution level is formulated using information on forecasted constraints and grid topology, this includes obtaining a distribution network expansion plan. Next, this thesis performs a technical/economic analysis of future distribution grids. Through co-simulation of electricity distribution networks and decentralised electricity trading platforms, an advanced methodology is developed for the assignment of electricity use of network charges. Ultimately, to paint a broader picture, this manuscript explores the socio-economic implications of the energy transition through the long-term simulation of access to distributed generation for small-scale participants at a national level. These new propositions are validated using two standard IEEE test networks, two real distribution feeders in the west coast of Ireland, and ultimately the entire interconnected distribution and transmission networks from Ireland. Technologies studied include small-scale rooftop solar PV, wind turbines, battery energy storage systems, voltage regulators and infrastructure upgrades. This work presents novel tools for planners to address the new challenges of modern and future distribution networks.