Legal Question in Real Estate Law in California

No

Assuming both property owners have been around, and have paid their property taxes, and that the properties are in a more-or-less urban setting, no. There once was a so-called "agreed boundary principle" where neighbors' express or implied agreement that a fence, hedge, etc. was the boundary would trump an old and perhaps inaccurate or vague legal description. The principle may still be applied occasionally nowadays, but only in really undeveloped rural areas. In recent decades, courts have shown a strong preference for using and enforcing property boundaries as written up in recorded maps and deeds based upon surveys and/or more or less permanent landmarks.

The doctrine of agreed boundaries requires the parties to be unsure of the true property line and to have actually agreed on another monument to identify the property line. A fence standing alone inside someone else's property line is insufficient.