I recently answered a Zillow question about why Zestimate home values in Castle Rock, CO are inaccurate for homes, condos and town homes. Here is my response:
I understand your concern and frustration. However I don’t believe you should worry over an automated computer model which uses a generic program to estimate property values. Zestimate is merely a tool. Very offer Zillow’s “Zestimate” is very wrong in it’s evaluations for many reasons. 
Quality, condition, subdivisions, amenities, age, lot, views, and much more. Case in point – I am working with a property right now and Zillow states an increased value of $100k+ in the past 6 months. Have you heard of any recent situations where a property in the $600k range has increased in value $100k in the past fews month? Probable not. Another Broker and myself have no idea how Zillow came up with that increased value.
The true value of a property is what a Buyer is willing to pay. Just because Zillow states a value, does not mean that it the true value. That applies to appraisals, Broker Price Opinion’s, Comparative Market Analysis’, and other valuation estimates. If you want to know what you home is worth, discuss the situation with knowledgeable Real Estate Agent who knows the local area, can review recent sold units, an understands the current market. That will be your best assessment for a valuation.
Someone writing a check to purchase your house, knows the value best – not a computer generated report.
Thank you,
Jim Garcia