The non-dispatchable and intermittent nature of renewable distributed generators, i.e., photovoltaic and wind power plants, can lead to voltage variations and disruption in energy exchanges, especially in multi-area active distribution networks. Therefore, to manage inter-area energy exchanges in the presence of high penetration level of renewable distributed generators, the flexibility and controllability of the grid should be increased by using soft open points. However, the investment cost of soft open points is very high. On the other hand, soft open points affect the energy transactions in the multi-area active distribution network, so, they should be optimally allocated with the satisfaction of all the network areas. In this research, for the first time, a new decentralized framework for optimal siting and sizing of soft open points is developed, considering the self-interested nature of the areas and preserves their autonomy and information privacy. By using the suggested framework, different areas can negotiate with each other on the location and size of soft open points, and the relatively equal share of their investment costs. The correctness of the proposed decentralized planning model is confirmed by two case studies in MATLAB. The results confirm that the proposed method can allocate the share of each area based on the profit from new SOPs and also preserve the privacy, in both case studies.