Assessor > actual sqft: 2500 vs 1800.

[Luke Mike]

Hello,

I saw a house listed on the MLS at 2,500 sqft. Assessor has the same tax records: 2,500 sqft. House is from 1950.
The actual sqft is only 1,800. The extra 700sqft is basement space and only 6ft tall. 

Obviously this wasn't up to building code, even in 1950. It sounds like the assessor made a mistake when adding the numbers. 

I'd like to buy the property and add to it. Obviously the city will find out the space is only 6ft tall. 

How does it work in this situation? Would they remove it from the total sqft, and bring the house down to 1,800sqft?

If we were going the other way around (assessor < real: unpermited addition -> permitted addition), the city would be asking me to pay a bunch of fees and back taxes. Do I get to ask the city to pay me back taxes in this situation? Ha, I can probably dream on this front.

California, Bay area.