Broadband Powerline Communications (PLC) is currently used in two application domains: Access networks and In-Home communications. Access networks are used by the service providers to provide network services to their customers, and In-Home applications are used by the users privately inside their households. In the situation where an Access network is connected to a household with an In-Home network, the PLC communication signal can propagate between both networks. Because both systems utilize the same frequency band, their transmissions may interfere with each other. As a natural characteristic of the PLC system, the user is not moving but their statuses and resource requirements are changing during the run-time. Some coexistence protocols are proposed, however, they are not designed for optimizing the available transmission capacity. Moreover, a distributed approach is required when it is not possible to organize a centralized controller for both Access and In-Home domains. Therefore, we propose a channel self-organizing protocol which allows both domains to cooperate in order to share and optimize the transmission capacity dynamically. Employing this algorithm, each system detects the interferences from its neighbors and organizes its resources to achieve the overall maximum channel utilization. On the other hand, the protocol maintains the balance of channels per user in different cells. The proposed channel allocation protocol is analyzed and evaluated by means of two example network models.