Zestimate

As mentioned previously, the Zestimate is a starting point in figuring out the true value of a house. The amount of data we have for the house affects the Zestimate accuracy. If your home facts are incorrect or missing, you can update your facts, which may affect your Zestimate value.

• Price history and tax history: Check to see if your price history (the sale price and date you bought your home) and your tax history are correct on Zillow. Depending on the area in which you live, this can be a big factor in your Zestimate. If data are missing or incorrect, please let us know. Click "Edit" on your home details page, then "Report Problem With Home."
• Updates and remodeling explained: Most upgrade information is not in the public records, and is not easily quantifiable. We do not know about home updates and remodels unless they have been reported to the local tax assessor, so those items are not used in Zestimate calculations. While we do utilize user-submitted data that is measurable, (e.g., additional bedroom count, bath count, and square footage) there is no way for us to systematically gather and verify the type of remodel or build information where the value is based upon how the final product appeals to the buyer. Because of this, the algorithm can't use that information.
• Your home icon on the map: Region boundaries usually do not affect the Zestimate. That being said, if the home icon is in the wrong spot, either Zillow staff or the home owner can correct that. Click here for the Move Home Instructions

Updates to your home facts will be factored into your home's Zestimate immediately. If the updates are not significant enough to impact the home's value, your Zestimate may not change. We refresh Zestimates for all homes daily. On rare occasions, this schedule is interrupted by operations associated with algorithmic changes or the deployment of new analytical features. 

The number of transactions in a geographic area affects how much we know about prevailing market values of homes in that area. More transactions provide more data and improve the accuracy of the Zestimate. Also, we use public and user-provided data for house attributes, and some areas report more data than others. The more attributes we know about homes in an area (including yours), the better the Zestimate. Remember that homeowners can also update their home facts if they feel they are incorrect or there are missing values, and the updates may affect the Zestimate value.

No. The Zestimate is not an appraisal and you won't be able to use it in place of an appraisal, though you can certainly share it with real estate professionals. It is a computer-generated estimate of the worth of a house today, given the available data. Zillow does not offer the Zestimate as the basis of any specific real-estate-related financial transaction. Our data sources may be incomplete or incorrect; also, we have not physically inspected a specific home. Remember, the Zestimate is a starting point and does not consider all the market intricacies that can determine the actual price a house will sell for.

We use proprietary automated valuation models that apply advanced algorithms to analyze our data to identify relationships within a specific geographic area, between this home-related data and actual sales prices. Home characteristics, such as square footage, location or the number of bathrooms, are given different weights according to their influence on home sale prices in each specific geography over a specific period of time, resulting in a set of valuation rules, or models that are applied to generate each home's Zestimate. Specifically, some of the data we use in this algorithm include:

Physical attributes: Location, lot size, square footage, number of bedrooms and bathrooms and many other details.

Tax assessments: Property tax information, actual property taxes paid, exceptions to tax assessments and other information provided in the tax assessors' records.

Prior and current transactions: Actual sale prices over time of the home itself and comparable recent sales of nearby homes

Currently, we have data on 110 million homes and Zestimates and Rent Zestimates on approximately 100 million U.S. homes. (Source: Zillow Internal, March 2013)

We not only have Zestimates for homes now, we have used massive computing cycles to go back in time to generate historic Zestimates as well. Sound hard? It is, but it's critical because it allows you to see how a home (or an area) has changed in value over the years.

Yes. When major improvements to the algorithm are made, we do re-compute the historical Zestimates for affected homes. Our purpose in doing so is to provide consumers with the best estimate of historical property valuations. A historical Zestimate is not like a historical stock price, which doesn't change after being recorded. A stock price is a record of an actual empirical event (and, as such, shouldn't change). A Zestimate, on the other hand, is an estimate of the market value of a home, and can change when we have a better algorithm to estimate that value.

Note: We never allow future information to influence a historical Zestimate (for example, allowing a sale in 2009 to influence a Zestimate in 2008). Each historical Zestimate only uses information (e.g., prior sales) known prior to the date of a given Zestimate.

Yes, a team of statisticians is working every day to make the Zestimate more accurate. Since Zillow's inception in 2006, we have deployed three completely new versions of the algorithm (2006, 2008 and 2011), but incremental improvements are made between major upgrades with new iterations being deployed regularly.

We refresh Zestimates for all homes daily. On rare occasions, this schedule is interrupted by operations associated with algorithmic changes or the deployment of new analytic features

No, the Zestimate is intended to provide an estimate of the price that a home would fetch if sold in a full-value, arms-length sale (e.g., the sale isn't for partial ownership of the property or between family members). Our extensive analysis of foreclosure re-sales (typically, but not always, real estate owned, or REO, sales) supports the conclusion that these sales are generally made at substantial discounts to comparable non-foreclosure sales. As such, these sales are not used by the algorithm to produce the Zestimate. That is not to say that foreclosure re-sales do not influence the Zestimate at all, as foreclosure re-sales do suppress the sale price of surrounding non-foreclosure homes, and the price signals from these surrounding homes are used by the algorithm to produce Zestimates in that area.

The Zestimate is created by an automated software process, designed by statisticians, and there is no ability for humans to manually alter the Zestimate for a specific property.

We do not delete Zestimates. We monitor customer feedback for systematic issues with the algorithm, but do not change individual Zestimates in response to customer feedback. The Zestimate is designed to be a neutral, unbiased estimate of the fair market value of a home, based on publicly available and user-submitted data. For this purpose, it is important that it be based on identical information about homes (e.g., beds, baths, square footage, lot size, tax assessment, prior sale price) and that the algorithm itself be consistently applied to all homes in a similar manner. This ensures that there is no preference for some homes relative to others nor are there valuations based on facts that are not accessible to all Zillow users. Some homes may be very unique in ways that are not well captured by existing data, and the Zestimate may be less accurate on these homes. To provide more data on your Zestimate, you can post your estimated value and comment in the Owners Estimate section indicating your support for a different valuation.

The number of transactions in a geographic area affects how much we know about prevailing market values of homes in that area. More transactions provide more data and improve the accuracy of the Zestimate. Also, we use public and user-provided data for house attributes, and some areas report more data than others. The more attributes we know about homes in an area (including yours), the better the Zestimate. Remember that homeowners can also update their home facts if they feel they are incorrect or there are missing values, and the updates may affect the Zestimate value.

Our estimating method differs from that of a comparative market analysis (CMA) done by real estate agents. Geographically, the data we use is much larger than your neighborhood. Often times, we use all the data in a county for calculation. So though there may be no recent sales in the "neighborhood", even a few sales in the area allow us to extrapolate changes in the local housing market. However, the data we gather does allow the models to incorporate the geospatial (neighborhood) patterns of recent sales.

Our accuracy depends on the home data we receive; see our Data Coverage and Zestimate Accuracy table to see how accurate we are in your area. When it comes to unique homes (e.g., luxury mansions, unusual designs) we are less accurate in our Zestimates.

It's important to remember that Zestimates track the market, not drive it. People ultimately have more fundamental reasons that drive what they choose to buy or not buy. Our data shows that half of all sales are generally above the Zestimate. To provide more data on your Zestimate, you can post your estimated value and comments in the Owners Estimate section. The purpose of the Zestimate is provide data in a user-friendly format to promote transparent real estate markets and allow people to make informed decisions.

No, you can't. To get a federally guaranteed loan, a law called FIRREA (the Federal Institutions Reform, Recovery and Enforcement Act) requires an appraisal from a professional appraiser. Without limitation, lending professionals and institutions are prohibited from using the services in making any loan-related decisions. The Zestimate is our estimate of fair market value, a starting point for home buyers and sellers and anyone just plain interested in the value of houses. You can use it in judging market trends, and in calculating all sorts of things for your personal purposes. Click here to view our terms of use.

If you see two Zestimates for the same property, you can click on Edit, Report a Problem, then choose incorrect facts from the drop-down menu. However, if you are a homeowner with multiple parcels, and this is the reason there is more than one Zestimate for your address, we match the parcels on record with the county. If you officially combine parcels with the county, they will send us the updated information.