Legal Question in Real Estate Law in California

First, why would the neighbor have to consult YOU? It's your mother-in-law's property; if anyone needs to be consulted, it is she, not you (unless you are her court-appointed guardian or something like that). Who is the "we" that shares the lawn?

Also, I have a problem with the concept that your mother-in-law "shares" this lawn. The neighbor who owns the lawn might consider someone else sprinkling his lawn as a trespasser!

My point is not that your mother-in-law's genuine feelings that, having cared for this lawn for so many years, it has become hers, at least in part. It is that the neighbor is the lawful owner and probably can build the fence.

There is a principle of "adverse possession" and a related concept of "prescriptive easement." Sometimes, neighbors acquire legal rights in property to which someone else holds title. Indeed, this MIGHT be such a situation. Adverse possession is unlikely, unless she can show payment of taxes on the lawn. Also, I think under recent decisions I've seen, watering a lawn and treating it as your own PROBABLY does not give rise to an easement to continue watering, or to continue any other related use....but in truth I cannot totally rule out a possible prescriptive easement in favor of your mother-in-law.

If she cannot reach a negotiated accommodation with the neighbor, she'll need to contact a local real-estate attorney who can delve deeply into the facts, perhaps look at the properties, and give her an opinion based on more information than can be processed through a bulletin-board type operation such as LawGuru.

I agree with Bryan and he probably doesn't have to consult you nor your mother in law if wants to build a fence on his property unless there are some restrictions in his deed or other agreement that there was to be a common lawn.

Good luck and hope this helps.