Legal Question in Real Estate Law in California

I'd be very cautious about the proposition that the parcel is build able. Occasionally one can legally build on a parcel that is significantly smaller than the zoning requires, but more often it's either outright impossible or entails an arduous application process. At a minimum, have a detailed conversation with a senior planning official before accepting the seller's version. As for the trees, I think it would be necessary to go to court and try to have them declared a private nuisance, which would be a rather long shot if they were planted and even longer if they are growing naturally. You might consider getting an opinion from a licensed arborist as to the hazard potential, but overall I suspect this isn't the right parcel for you.