I agree with most of what you said, except for the last sentence. I agree that you cannot intentionally cause water to drain into the next lot. for instance, you cannot put a ten-foot extension on your downspout and point it directly at your neighbor's house. Nor could you channel downspout water through underground pipes that exit right at your property line and point at a nearby neighbor's house. However, there is no legal provision, nor should there be, for water running downhill. That may not be what you meant, but that is what you insinuated. I live on a corner lot, and the side street runs downhill. All of the rainwater that does not soak into my side yard runs downhill into the neighbor's yard to the east, and his runs downhill to the east and so on. I have NEVER heard of any law or ordinance that states that rainwater is not allowed to run downhill and 'drain onto an adjacent property.' For sure, you cannot intentionally divert water (for example from downspouts) at your neighbor, but water does run downhill.