Neighbor’s Roof Run Off Is Soaking My Exterior Wall And Causing Interior Water Damage

My neighbor built an extension to his house, and I believe the rain is running off it and onto my exterior brick wall. There is water damage to my interior wall and ceiling. From the discoloration of my brick, it seems pretty to clear that the rain comes off his extension, does not have proper gutters or a downspout. Neighbor, however, still claims he is not the source of the water.
This is in New York City, so What are my options?
Mechanical:
- Waterproof my exterior wall, which I will need to reapply every two years
- Attach an awning to redirect the water to his house
- Offer to pay for his gutters
Legal
- Call Dept of Buildings as his extension does not have a certificate of occupancy
- Call our homeowners insurance and let them deal with the neighbor
- Lawyer up and video water coming off his extension on to my house
Any other ideas?
The neighbor is not very reasonable or rational. He has so much to lose: his home's (illegal) extension (if it is indeed not up to code/permitted/lacks a certificate of occupancy), rental income (he doesn't live there and rents out the house), and other discoverable violations when the inspector comes.
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