Retaining walls are a common wall if they sit on a boundary. Both parties are responsible to maintain it. Whoever told your neighbor that it is your responsibility because you are on the lower side is only acting in the interest of their client. As a building and zoning officer for 14 years this matter came up frequently. My job was to try to resolve these issues. Many times there was no way to know which house was built first. Most new homes have a retaining wall put up to keep the upper yard from sliding down into the lower yard. Think about it. Why would a lower property owner put up a retaining wall? The answer is "to keep the yard from falling up"? No. It doesn't work that way.
As a matter of fact, it is against all building codes to have roof downspouts on one property from draining onto another person's property. The same principle applies for the retaining wall. If debris from their yard is ending up in your yard through erosion they are, in effect, damaging your property and you have the right to sue them for the damages. You may want to discuss this with them and if they still refuse to co-operate go to your local building inspector's office and file a complaint. They will investigate it and order the upper property owner to stop allowing debris from going onto your property. This is a free service you may want to tap into. That way there you don't have to litigate. Why should you spend your hard earned money on something like this anyway? When they come to you later for half the bill maybe you will refuse and tell them that you had previously offered to help but they refused so you spent the money on something else.
Building Inspectors hate dealing with common retaining walls because they are a pain to resolve but when it comes to it, they have to enforce the laws whether they like it or not. You may also want to get a land surveyor to determine exactly where the line is. This however costs money and your neighbor can get their own surveyor to show that the wall is on your side. In a case like this an impartial surveyor measures where the line is and his ruling stands as he will probably side with one of the two surveyors. Just contact your local building department first and take it from there.
Hope I Helped :)
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