Legal Question in Real Estate Law in California

If they have artificially diverted the water flow, you may have legal remedies. If it is the natural flow of the run-off, you may have a claim against the developer/builder of your house if it is new enough.

Your rights depend upon whether the flow is natural or artificially concentrated in some way. If the flow is pretty much natural, i.e., no dams, dikes, diversion canals, outlets from swimming pools, no drainage ditches etc. involved, you're stuck with it -- water flows downhill. If, on the other hand, the uphill neighbor has done something to add to the natural volume and/or velocity of water runoff, his liability to you for damage done by the runoff will depend upon the overall reasonableness of what he's done. An uphill neighbor's liability for "unreasonable concentrated discharge of surface waters" is discussed by the California Supreme Court in the case of Keys v. Romley (1966) 64 Cal.2d 396.