Hi, if I understand you correctly, then when you place a building you want to notify/call an event on all buildings that are within a certain radius.
Then I would just loop through all buildings you have on the map and check the distance and if it is closer than the certain radius you want, call an event on them. Or if you want to do this via event dispatchers, then every time you place a building call an event dispatcher that all buildings are listening to (e. g. some building manager) and then check the distance and if it is closer than a certain radius do something.