We've discussed this a few times on the PB forum before. The laws vary from state to state, but there are instances where a BOW touching one landowners property gives that same landowner legal right to the entire BOW. The best decision IMO is to keep 100% of the water captured within your property lines.
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There is a little bit more that should be considered...
Even if you do keep the water captured within your property lines, how will it affect the adjacent soils on your neighbor's property? When you get a heavy rain, will the rising water level encroach? Even during normal pool height, could the saturation compromise the soils on your neighbor's property? ie; his driveway runs close to your pond and now the soils are soft....or something similar.