City Building zoning is R1, county tax roll says Duplex ?

[Jo Zhou]

Hi, I am buying a house in California as an investment property. This house is located in a single family house zoned area, but with a separate 1 bed/1bath unit in the back attached to the front house. The front house is a 4 bed/2 bath house. I am looking for buying a duplex, but in our city, the multi family housing area are really not good area though. Some decent multi family duplex is easily well above $900k. This one is listed for $730k, I think this might be a good option. 

So I did some research, then I got this weird results:

1. city building records has a bedroom, a bath addition, permitted work. It didn't say it was a 2nd story addition. The addition was done on 1977. Would the city ask me to tear it down if they saw it.  But the work was inspected by the city, how come the city didn't see it was a 2nd story addition?

2. The 2nd unit kitchen was unpermitted for sure. Would the city ask me to tear it down if they know it?

3. The most odd thing is that the county tax roll says it is a duplex with the correct numbers of rooms and baths. (Not sure about kitchen, the county didn't release the info to non-owner). It means the property tax is paid correctly over the years.

4. The house changes hands several times. The appraisal always included the 2nd units in the value. So my lender says I will be fine with my loan just because the tax roll says it is a duplex, so the lender can count 2nd unit's income. If the tax roll didn't say it is a duplex, even if the 2nd unit is rentable, the lender will not count its income. It seems the lender didn't care it is permitted or not, it just follows the tax roll. Is my guess correct?

So right now, I got so confused: am I buying a duplex or not? Whose authority will be higher: county tax roll or city building department? Should I buy it as an investment? What is my risk? Would the county assessor change it back to single family house? Could I legally rent it to 2 different family? (The worst case, I plan to move in, then rent it out to get a roommate in the 2nd unit since the house is located in a very nice area.

Please help me with your experience and knowledge. Very appreciated.