input from owners

I am wondering just how much or how little value your estimate places on information about the home from home owners.  In my case, I was already half owner of a home when I purchased it from the other partner.  Immediately the zestimate plummetted and in spite of increased improvements to the property as well as high values of sales in the neighborhood, it continues to decrease.  In my opinion, with the current landscaping and fencing projects alone, given the recent sale of a neighbor and the assessment for the bank, the house is worth at least 490,000.00  and the zestimate is coming in as between 204 K and 346K.  This is in spite of my having indicated my purchase price represented only 1/2 of the value.  Do others out there have that experience?  Is there some way I wonder of getting more closely connected values?