How Close Is Too Close?

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A property developer built houses near my house in the backside. I currently own 2.06 acres and saw that he is now creating a walking trail extremely close to my property. My question is if there's a distance or a few feet that he is required to stay away and should I have been notified about this how that people are going to be close to my property and have full access to my back yard since there's no fence?