If I have fencing around my yard, but it was all erected by my neighbors around me, can I use that fencing as the enclosure for an 'Eiruv?
Of course, CYLOR for practical cases. I'm asking for the basic rules as recorded in Halachic literature.
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The Shulchan Aruch rules (OC 362:3) that a mechitza which came into being by itself counts as a mechitza at least for areas less than the amount of land you could plant 2 sa'ah of grain on (much bigger than your average backyard). So clearly at least for small areas like your backyard, the details of the mechitza's identity do not matter.
Furthermore, I have worked on and repaired eruvin in a positive integer number of cities and have seen many places where the boundary utilizes existing edifices that were in no way owned by Jews or built for the purpose of the eruv.