Dear Customer (name blocked for privacy):

CA law requires sellers and real estate agents to make many disclosures to buyers of homes, which you can see listed here http://www.wrightrealtors.com/disclosure_forms.htm and here http://www.dre.ca.gov/disclosures.htm

Normally, courts have also required sellers or real estate agents to disclose any "material" facts about a property to buyers, particularly if the buyers ask about such fact or such fact or if such fact is misrepresented or failed to be disclosed to the buyer/potential buyer when a reasonable person would think it should have been disclosed. Clearly, where the disclosure is not one required by a written law, but is instead one that requires court determination of its being material or made under "reasonable person" guidelines, there is no set answer.

Normally, disclosures are only required about the property being sold itself (such as about its physical hazards, problems, etc.), not about any problems others may have or claim to have with the property. However, if you feel the problem with the neighbor was purposely hidden or misprepresented to you, you may have a claim against the former owner and/or their agent for any loss or damage you believe you may have suffered due to this.   

You should speak to a real estate attorney in CA about this matter to see if they can help you with the neighbor and/or the sellers/agents involved.

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PLEASE NOTE: Responses here are for information/education only, NOT legal advice and do not form attorney-client relationship! Only licensed attorneys you hire in your state can provide legal advice.