Re: Fence Dispute in San Diego
Start with reference to CA Civil Code §841.
841. Coterminous owners are mutually bound equally to maintain:
1. The boundaries and monuments between them;
2. The fences between them, unless one of them chooses to let his
land lie without fencing; in which case, if he afterwards incloses
it, he must refund to the other a just proportion of the value, at
that time, of any division fence made by the latter.
You need to determine if the existing fence between you is a boundary fence for purposes of this code. Note that the law does not require that a boundary fence, for purposes of maintenance responsibility, be precisely on the property boundary line. Is her property otherwise similarly enclosed? If so, she and you share joint responsibility for the upkeep of the fence. That doesn't necessarily mean that she would need to incorporate changes in a new fence/wall to alleviate the water problems (depending on other particulars of your situation), but it does obligate her to pay half the cost of repair or replacement of the fence using materials similar to what it consists of now.
It might be apparent to you, but is it obvious that the water causing your problems is coming from your neighbor's property? Would it be obvious as shown by photos (something like dirt or other debris washed across and deposited on your yard, washouts like mini stream beds leading from her yard toward your house, etc.)? Was one house built earlier than the other or were they built about the same time? If your house was there first, do you have any way of demonstrating that you did not have problems with water pooling under the house prior to your neighbor's house being built? If your house came later, then the waterflow may have been a pre-existing condition that you will have to solve without her cooperation if she no longer wishes to participate. If they were built about the same time and there was been no additions to her home or landscaping which would have changed surface flow conditions, then the problem would have existed since the homes were built and the developer should have been responsible for correcting the condition. Since it's been long enough that the fence is not collapsing, it's well beyond the statute of limitations for such construction defects, so it's too late to have the developer deal with it.
If it is important to you to determine who is responsible for the water, you may need to hire a civil engineer to be able to show by reliable and objective means where the water causing your issue comes from. But the cost of that might be as much or more than just installing some french drain near your side of the property line to catch and divert the water.
I'm a little concerned about your statement that the "new property lines actually fall onto my existing property". When surveying an existing property (as opposed to creating new lots by dividing a property), the surveyor's job is to locate the lines in the same location where they were originally established. Is your statement based on a presumption that the existing fence had been built on the property line, or did you have some other basis for your belief of where the lines of your "existing" property were?
I'm a surveyor, not your surveyor & not an attorney.
Advice is general survey, not legal. Hire a local professional for specific advice.