Owner Renting my Land As a Parking Spot

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Curious to get some opinions on this. I own a duplex in the Bay Area that's zoned for three exterior parking spots. The adjacent property has the same lot dimensions, and it's also zoned for three parking spots. The adjacent property owner has been renting four spots out. When asked about it, he basically played dumb. I sent him the legal zoning documents, and they clearly show both properties with the same width and callouts for three spots. Basically, he's squeezing a fourth spot that's in between our two properties, so technically I own half of it.

I am obviously not happy. It's not an insignificant amount of money when you're dealing with Bay Area parking. Any ideas on how to proceed?