Legal Question in Real Estate Law in California

You are unlikely to find any local law on the subject, since state-wide law applies, though there may be a local building code that enforces it. The law itself is older than California, older than the U.S. even. There can be no liability for natural runoff flowing its natural course to another person's property. However, once you gather, concentrate, and/or channel it, you become liable for nuisance and/or trespass if it flows onto another person's property and causes damage or interferes with the neighbor's use of their property. That principle goes back to old English Common Law which became part of the common law of this state to the extent not contradicted by legislative statute, when California became a state.