This paper presents a methodology to assess the cost of upgrading the LV network so that it can host increasing levels of PV distributed generation. The cost is valued in terms of investment, operation and maintenance and losses. A real scale case study (80,000 customers from urban and rural areas, 460 GWh of demand and 3,600 km of MV&LV lines/cables) is thoroughly analysed, using the actual network topology along with the hourly demand of each supply point, provided by smart meters. With the help of a planning-oriented load flow solver, technical violations (congestions and overvoltages) are quantified and classified for different PV penetration scenarios. Then, a heuristic procedure is implemented aimed at identifying the most cost-effective investment solution that sequentially eliminates the previously ranked excessive congestions, assuming that mild congestions and other operational problems, can be addressed without extra investments, either through network switching practices or through the implementation of local flexibility markets. Specifically, it has been found that the direct costs (investment and O&M) arising from the massive deployment of PV generation amount to 2.73 ?/kWp and 10.18 ?/kWp for scenarios with 30% and 50% PV penetration levels, respectively. No additional investments needed in digitization, monitoring, automation and systems have been considered.