Why is zestimate way be price range? 2 houses - on 6.02 acres (totally renovated)

Hi,
I recently posted my property with zillow but was shocked at zillow's estimate.  In 2005 the property was sold at 800,000.  The owners had planned to redo everything and pretty much tore it apart.  I came in in 2008 with nothing but exterior walls.  I put in new everything, wiring, sheetrock, etc.  Pretty much renovated the whole place.  another concept which is not noted is that there are 2 homes on the property, one of which is rented.  This is not stated in the value as the old assessment.  Now I don't want to run down and have my home reassessed at this time (cause my taxes will be overkill)  Please update my zestimate as this is working against my sale.  Thanks & feel free to discuss this with me...