Zestimate HISTORY changed?!?!

I don't know the details, but they revised their algorithms around Sept-Oct 2007, and retroactively re-calculated all their historical estimates. You might try searching for this; it has been discussed before.

Hi Kevin.  A new Zestimate algorithm was put into place in January 2008 and we re-computed all historical time series at that time as well.  We don't plan to re-compute historical series very often (indeed, January was the only time we've done it since launch in 2006) for the very reason that you outline here: it can be disconcerting to some users when history becomes not so "fixed."  We do so, however, when there are major changes to the algorithm which we think will give our users a better sense of historical values both on homes (through Zestimates) and regions (through the Zillow Home Value Index). We promise these changes will be few, far between, and well publicized. 

In the case of your particular home, it looks like the changes made to your home facts (specifically, lowering the lot size by some 4,000 square feet) resulted in a lower valuation when the Zestimate history was recomputed (because when the Zestimate history was first computed, the original home facts were in place). Thus, the original valuation (using orignal facts) on the date of purchase was $560K, the new valuation (using revised facts) on that date is $475K and, in both cases, the Zestimate bumped up to the price you paid once the algorithm had knowledge of the sale (note, when recomputing history, the algorithm will not, by design, consider the sale price in valuations made before the sale date). 

Anyway, it sounds like you think the original valuation (with more square footage) was a more realistic price at the time you bought your home.  Taking the price for which the home sold prior to your purchase (sold for $346K in Aug 2003) and adjusting it by the change in the Zillow Home Value Index between Aug 2003 and June 2006 (when you bought it), one arrives at an adjusted prior sale price of $552K which is higher than the Zestimate at that time and closer to the price that you actually paid for the home. This might suggest that our models were putting too much weight on the models relying on home attributes relative to the ones relying on prior sale price (prior sale price becomes less influential in the mix of models around the three-year mark after a home is sold).

Hope this helps.

Well look like you explained why you messed it up , but not what you intend to do about it.

The OP clearly stated that the new values distort the real world pricing.  and shows new users a FAKE history.   Now when they try to sell it, anyone viewing it will be MISLEAD. 

So how are you guys going to fix it , to make it clearer to user that the history is just fake junk you made up to suit a new model?????

Thanks for all the comments, especially the explanation by Stan...

I'm not sure how I feel about this.  It seems to me if the Zillow algorithm changes, the data displayed to users should reflect the before and after algorithms (or there should be a big disclaimer saying the data has been changed).  One of the reasons I purchased the house in the first place (at the price I paid) was because Zillow told me that the house price was 5% above the Zillow value (which was consistent with other Zillow values relative to sales in the area at that time)...(again the house had 5 offers on it when I purchased it...near the peak of the housing bubble...sigh)

But again, the "housing bubble" data is completely lost to anyone coming in to look at the "history" of the value of my house if I go to sell it.  They'll look at the purchase price as "way out of whack" and it could be a real problem for me...

Anyway, I don't really expect Zillow to change anything...

I guess I've just learned my lesson: You really can't count on any single source of information as "accurate" even in this day of Super Crunching algorithms and seemingly limitless data...

At the end of the day the "accurate" value of the house was the price that I paid for it (as 4 other people were all willing to pay that same price at the same time).  Since the alogorithm does adjust to the purchase price, that's really all I can expect.

Thanks again for everyone who commented on this thread. :)

-Kevin