I bought a house in Los Angeles county in March 2009. It's on a flag lot and I have an easement to use the driveway that's owned by the neighbor in the the front. Once the driveway reaches the property line the driveway becomes mine. My neighbor to the side is also on a flag lot and must use the same driveway to get to his lot. He purchased his lot about 2 years ago and built on it a large 2 story house close to the side property line. When I bought my place I assumed he had an easement to access his place. He only has an easement through the front 200 feet but not through my property. He either didn't do his homework to find out that his lot is land locked or most likely since he's a bit of a bully, nutcase, figured he could get away with it. If I had owned my property before he built I would have stopped him but what are my rights now after the fact. Needless to say, It would be more pleasant and private having an empty lot next to me rather than a large house looming over me inhabited by an agressive alcoholic jerk.

You will need a local title expert to look at the plot plans to give you a better answer here. Is it possible for you to contact the title agent or attorney that closed on your purchase?

Good advice from Mass Mortgage.  If it turns out that he doesn't have legal access then he or the title company that insured his property might have to buy those access rights from you.  Check with an attorney about filing a lis pendens against his property if he is uncooperative.