Legal Question in Real Estate Law in California

I think your first step ought to be an accurate determination of where the property lines are. My suggestion would be to get a couple of bids from local surveyors with experience in doing urban lot-line locations for possible litigation needs. With modern techniques, surveys are relatively inexpensive and pretty accurate, especially when the surveyor is working in familiar territory. Provide the surveyor you select with all the information including parcel maps, but most important, the legal description from your deed.

There are some legal theories that might be worth investigating if the survey results are unfavorable, These include 'agreed boundary' and 'easement by prescription' and others, but in an urban setting judges are more likely to adhere strictly to the deed descriptions.

Although I agree with Mr. Whipple that you should get a survey to know where the exact legal property line lies, he seems to be forgetting that you are dealing with a public entity owner on the other side of the line. You can't acquire public property by adverse possession or any other equitable method the way you may be able to do between private owners. So whatever the survey shows will be what you own and don't own, end of story.

I agree with Mr. McCormick, in that you cannot gain title to property from a public entity by way of adverse possession.

You can't rely on a tax assessor's map to figure out where the boundaries of your parcel are. You have to rely on the boundaries as set forth in the legal description in the deed by which you took title, and have it surveyed.