My neighbor would like to extend his house illegally into the backyard. For various reasons such as, reducing the air and sunlight in our backyard, I am not happy with this undertaking. In the past I wrote to him that I don’t allow him to build this extension and I would notify the authorities of his intentions. Am I permitted to inform the building dept to prevent him from building?
A dispute between Jews must be resolved in a Beis Din not a secular court or authoroties. If the other side doesn’t want to come to Beis Din or doesn’t listen to their psak, the Beis Din will then give you a heter to go to the courts or do as you please. Similarly in your case you should first summons him to a reputable Beis Din. Behatslocho.
Choshen Mishpot 26:1, Rashi beginning of Mishpotim.