Gov. Rick Scott’s 4,900-square-foot house on Gordon Drive in Naples, as seen from the beach. Online real estate sites such as Zillow and Trulia offer widely varying estimates of the property’s value. Scott paid $11.3 million for the house, on 1.25 acres, in 2003. Gordon Drive, across from Naples’ tony Port Royal section, features some of the most expensive real estate in Florida.

Last Modified: Thursday, July 10, 2014 at 2:08 p.m.

Florida Gov. Rick Scott might own a teardown in Naples depending on which online home evaluation website you consider.

Trulia.com’s automated valuation model (AVM) estimates Scott’s 4,900-square-foot, beachfront house on Gordon Drive one of the priciest stretches in all of Florida is worth $5.3 million. Based on area land values, that would mean the house is basically worthless.

Zillow.com, by contrast, Trulia’s biggest competitor and the dominant brand in free online home valuations, puts its Zestimate of the Scott property at $17.4 million.

CoreLogic’s Real Quest Express splits the difference. It values the Scott property at $7.25 million.

Collier County Property Appraiser Abe Skinner, meanwhile, set its market value at $13.13 million as of Jan. 1, 2013. That valuation was likely influenced, however, by the spring 2012 sale of a new mansion on four acres next door to Scott’s house that changed hands for $47.2 million.

For the record, Scott and his wife, Ann, paid $11.3 million for their home, then 2 years old, and the land it’s on in 2003.

As consumers flock online with greater and greater frequency to value properties and make offers, sites like Zillow and Trulia have gained power and credibility.

But significant and often unexplained value disparities like the one involving the governor’s house on the popular websites rankle real estate agents, appraisers and homeowners alike, who contend that values cannot, or should not, vary so widely.

I think Zillow is too low, said Franklyn Skidmore, a Zillow-watcher who, along with her husband, Rodger, have listed their waterfront home on Siesta Key for sale at just under $2 million.

Zillow estimates the Skidmore home to be worth $1.384 million. CoreLogic values it at $964,000.

Still, many real estate professionals say fine tuning by the various sites especially Zillow have improved valuations greatly in recent years.

The models used to generate Zestimates over the last nine years have definitely improved, said Krystal Kafka Dunk, a senior residential appraiser who oversees mortgage appraisals at IberiaBank in Sarasota. I’ve seen Zillow and Trulia value estimates, web-based automated valuation models or AVMs, go from absurd to surprisingly accurate in some cases.

Real estate agents, too, say the online sites have tweaked their mathematical algorithms significantly, and for the better.

It’s a lot better than it was, said Realtor Catherine Seress, of Coldwell Banker in Sarasota. It might be because the market has stabilized, or that they have done something to make it better.

Zillow maintains it has a team of no fewer than 10 Ph.D.s made up of top-notch mathematicians and statisticians who do nothing but tweak the valuation models that compute Zestimates, said Zillow chief economist Stan Humphries.

We are working on our fifth version of the algorithm, Humphries said. Where we do miss, we fix that miss until it is as accurate as it possibly can be. We are investing heavily in making it better, constantly turning that crank.

Scott isn’t the only well-known Southwest Florida homeowner whose property valuation on the various online sites sways like a palm tree in a hurricane.

U.S. Rep. Vern Buchanan’s Longboat Key mansion has carries a Zillow Zestimate of $9.5 million, but Trulia considers it worth just $4.45 million, according to the sites. CoreLogic is off even more; it values Buchanan’s property at $3.01 million.

Sarasota County Property Appraiser Bill Furst put the value at $6.5 million, though that figure is likely to increase when new tax assessments come out next month.

Buyers, however, might take heed to the county in this case. That’s because the least expensive beachfront residence currently listed for sale on Longboat Key is $6 million.

Free for a reason

Looking up home valuations has become among the most popular exercises on the Internet, to no surprise.

It’s both free and easy.

Valuations are computed from municipal tax records, MLS listing information and price trends of properties that have recently sold. Key facts include the number of bedrooms and baths and the heated square footage of the home.

Trulia, CoreLogic and Zillow can all offer their information gratis because, as they are the first to admit, they are not real estate firms. Instead, they are media businesses that make money selling advertising and by linking customers with Realtors and other professionals.

To further improve, Zillow has started using geospatial information to assess whether a waterfront house is on the Gulf of Mexico or merely a man-made retention pond, Humphries said.

Seattle-based Zillow’s computer creates 1.25 million calculations nightly to formulate values, and they are reset each night, the economist said.

In all, the company maintains information on 100 million properties, about twice that of Trulia. That’s why a Trulia map search contains many properties for which there is no estimated value provided.

As will all computer estimates, though, the output is only as good as the input. Even frequent users and proponents of the sites acknowledge that there are limitations.

The various valuation methods’ shortcomings become particularly apparent when data is flawed either by lax recording and reporting of sales by municipalities; a lack of information about improvements to interiors and overall condition; and a lack of information about nuisances the fire station or strip mall next door, for instance that can impact value.

In some states, too, sales data is not disclosed, which skews valuations of nearby properties.

At the same time, unique homes such as historical houses, architectural gems or beachfront mansions are difficult for computer programs to evaluate. In considering them, Zillow relies on nearby sales and tax assessments as key indicators of value, Humphries said.

Houses in rural areas are hard to value accurately, as well, in large part because there is often a lack of available comparable sales, said IberiaBank’s Krystal Kafka Dunk.

Condominiums and townhouses present special challenges, too. While units are often the same size, each must be measured on individual interior upgrades, as well as on views and other accoutrements.

We live in a 28-unit townhouse community, said Andrew Chasen, a former Sarasotan who now lives in Richmond, Virginia. He checks Zillow often because he’s interested in seeing what other nearby units have sold for.

But he also argues that Zillow’s estimate for his house is way off.

We have done major upgrades that Zillow would have no way of knowing about, Chasen said.

The online sites’ models are most accurate when evaluating similar homes in subdivisions, say market professionals.

Margin for error

Another strength of online valuations is that they are real-time and not up to 18 months old, as county tax assessments can be.

Perhaps best of all, as far as consumers are concerned, they are free and provide information on homes with a few clicks that would have been difficult to gather in the pre-Internet era.

But even the online companies themselves acknowledge that there’s a margin for error.

We measure accuracy by comparing actual sales prices for recently sold homes over a three-month period with those homes’ estimates at the start of that three-month period, Trulia notes on its website.

For each county, we report the median error of these differences and the percentage of sales where Trulia Estimates were within 5 percent, 10 percent and 20 percent of the final sale prices. Since our release, we continue to work to improve our accuracy.

The models are designed such that the evaluation is systematically unbiased, which means just as many sales will occur above the Zestimate as below the Zestimate. It is challenging in a quickly moving market, when appreciation gets absurdly high, Humphries said.

When markets are growing extraordinarily fast, and you pull in prices from a couple of months ago, it is a fundamentally different price point and creates challenges in our models.

Zillow’s median error rate is just under 7 percent for valuations, though the company points out that that rate is half of what it was when the site went live.

Dan Gaertner, vice president of technology for Homes.com, says his site is typically within 5 percent of actual value.

Local real estate professionals, however, maintain that all of the sites have a fundamental flaw that will never be overcome with mathematics.

The problem is, it’s a computer that has not seen the properties, said veteran Sarasota real estate appraiser Howard Greber.

A home on Eagles Flight Way in North Port’s Heron Creek neighborhood, for instance, sold for $378,000 this year. Zillow values it at $393,000, Trulia says it’s worth $337,000, and Homes. com maintains it’s worth $379,300.

The online valuations of many luxury homes, however, are often divergent. Take the 10,000-square-foot house that sold for $10.25 million in late May at 4300 Higel Ave. on Siesta Key, for instance.

Zillow’s Zestimate put the value at $8.65 million. Trulia says it’s worth $4.2 million. Homes.com has the place valued at $4.8 million, while CoreLogic contends it’s a $3.225 million house. The county property appraiser listed its value at $4.47 million as of Jan 1, 2013.

Unusual houses hang it up, there is no way, Howard Greber said of the online sites’ accuracy.

Real estate professionals believe houses in fact, all property is worth what willing, arms-length buyers will pay, regardless of valuation. Such was the case on Higel Avenue, when Jeffrey and Susan Jones, of JCI Jones Chemicals, acquired it.

Love-hate relationship

Whether they admire it or despise it, most real estate professionals have resigned themselves to the notion that such online sites are here to stay, and have forever changed how real estate is considered by buyers and sellers alike.

It is not a demon, an evil, even though the information cannot be deemed to be accurate, Coldwell Banker’s Seress said.

It might only be ballpark, but it is handy and useful. The little graphs are nice for tracking the market.

Zillow has proven lucrative on another front for her, too. Seress bought some Zillow stock, and has watched it rise 35 percent this year.

Back in Virginia, Chasen has been using Zillow to explore a particular Sarasota neighborhood where he is considering buying a second home.

It is fairly important, said Chasen, who owned Chasen Galleries on Sarasota’s Palm Avenue from 1994 to 2002. Though I think of it only as a guideline. You can’t treat Zillow as the gospel.

Like many users, Chasen said the price estimates can elicit emotional reactions.

If it is favorable, it is good, he said, and if it is not favorable, it is not good.

Most websites echo Trulia.com’s caveat.

If you’re looking for a precise value for your home . . . we recommend you talk to a real estate professional. After all, our algorithms are smart, but we can’t know everything about your home’s features and location.