>I wonder what would her argument be if Zillow simply reported the City/County tax assessors estimates which are usually even lower than Zillow's.

County assessors for tax purposes are explicitly not trying to determine market price. Zillow should ignore it.

I'm not sure what you mean.

In Illinois (the state from the article) the assessor is calculating the market value:

Most real property in Illinois must be assessed based on its value on the open market, or its “market value.” This value is the amount at which a property would sell in a competitive and open market... [1]

[1]http://tax.illinois.gov/Publications/LocalGovernment/PTAX100...

Then it likely varies from state to state - or county to county.

See http://www.investopedia.com/articles/tax/09/calculate-proper... for different methods used. As an example, some places merely look at the cost of replacing the house if destroyed.

That's not how it worked where I grew up in Upstate New York. The assessed value was a number way below market value, but assessed values among different houses pretty much differ by the same proportions as the actual market values. I do not know why it is the case.

Tax assesment is useless in california?

Yes, because of Prop 13, assessments have very little connection to current market value.

Zillow does/did record that information, though for my house it hasn't been updated (at least on the public website) since 2014.