Zillow just accumulates data. If they have a ton of recent sales for your area, it will usually be fairly accurate (within a couple percent). If you don't have that much activity or are looking at an unusual property, then zillow can be wildly inaccurate.

Not very accurate at all.

All they have is data on prices and location to judge comps.
I live in an infill development of about 40 houses surrounded by much smaller and less expensive ones.

The Zillow prices are always way off.

The older houses in the area get knocked down to build houses comparable to my development.

Based on my experience, most of the time the "zestimate" was off. You have to realize the estimates are all computer generated based on the data. And the "data" doesn't include new carpet, upgrades, or the fact that the house is located on corner of a busy street.

Not very, especially if you have not sold your house in the last 10 or more years.

Zillow's zestimates are just an average of recently sold homes of similar sq footage and neighborhood.

We sold a couple of years ago, at a higher price than their zestimate, as we had a prime lot plus had remodelled, and nice, mature landsaping. Same model, several houses down sold 20% less.

It was on the opposite side of the street (no view) and had original flooring, cabinets, appliances etc. not much curb appeal either!

Buyers do take everything into consideration.

"Zillow's zestimates are just an average of recently sold homes of similar sq footage and neighborhood."

The "neighborhood" is based strictly on proximity, nothing else.

If they hit the correct number it is as much by accident as intent.

"If they hit the correct number it is as much by accident as intent."

I don't think that is fair. If you live in an average house, in an average area, with lots of recent sales, then the averages zillow kicks out will be pretty close to your home's value. That is just how averages work.

If you neighborhood has a wide range of house sizes, lot sizes, renovations, etc or if your immediate neighborhood is substantially different from the surrounding neighborhoods, then that "averages" don't make much sense to look at. Basically, by definition, if your house isn't average, then the average price doesn't apply.

Well, the appraisal on our house went fine and we close next friday. Thanks for all the advice :)

Zillow does not and cannot do anything but average, and if the houses are not uniform the average is not likely to match the value of a large number of houses in the group.

Zillow makes no attempt to actually figure out what are reasonable compos except they are located near each other.

Its attempt to value a particular property is just a crap shoot.

I would call that an accident.