The number of mortgaged residential properties in the United States considered "equity rich" hit a record high point in the third quarter of 2018, according to the housing data company ATTOM Data Solutions.

Approximately 14.5 million homes were defined as equity rich. This occurs when the combined amount of loans secured by the property is 50 percent or less of the home's estimated market value. This tally was a year-over-year increase of more than 433,000, reaching the highest level since data became available in the fourth quarter of 2013.

The equity rich homes represented just over one-quarter—25.7 percent—of all mortgaged properties during the third quarter of the year. This was up from 24.9 percent in the second quarter of the year, but down from 26.4 percent in the third quarter of 2017.

"As homeowners stay put longer, they continue to build more equity in their homes despite the recent slowing in rates of home appreciation," said Daren Blomquist, senior vice president at ATTOM Data Solutions. "West Coast markets along with New York have the highest share of equity rich homeowners while markets in the Mississippi Valley and Rust Belt continue to have stubbornly high rates of seriously underwater homeowners when it comes to home equity."

California had the highest share of equity rich homes, with 42.5 percent of mortgaged properties falling into this category. The state also had a high share of equity rich properties on the metro and ZIP code level. Of the 98 metropolitan statistical areas included in the report, three California communities had the highest share of equity rich properties, led by San Jose at 73.9 percent.

Among the 7,290 ZIP codes with at least 2,500 properties analyzed for the report, 417 had more than half of their mortgaged properties falling into the equity rich category. The top five ZIP codes with the highest equity rich share, ranging from 85.7 percent to 87.1 percent, were all in the Silicon Valley area.

Hawaii had the second highest share of equity rich homes at 39.4 percent. This was followed by Washington (35.3 percent), New York (34.9 percent), and Oregon (33.6 percent).

Conversely, ATTOM Data Solutions found that 4.9 million mortgaged homes were seriously underwater. In this situation, the total amount of loans secured by the property is at least 25 percent higher than the home's estimated value.

The share of seriously underwater homes stood at 8.8 percent of all mortgaged U.S. properties. This was down from 9.3 percent in the second quarter of 2018 but up from 8.7 percent in the third quarter of 2017.

Louisiana had the highest share of seriously underwater homes in the nation, with more than one in five mortgage properties—21.3 percent—falling into this category. Baton Rogue was the metro area with the highest share of seriously underwater homes at 20.7 percent, while New Orleans had the third highest share at 18.6 percent.

Mississippi had the second highest seriously underwater share at 16.2 percent, followed by Iowa at 15.5 percent and Arkansas at 15.3 percent. In addition to the Louisiana cities, the metro areas with the highest share of seriously underwater homes included Youngstown, Ohio (18.7 percent), Scranton, Pa. (18.3 percent), and Toledo, Ohio (17.7 percent).

More than half of mortgaged properties were seriously underwater in 26 ZIP codes, including ones in the metro areas for Atlantic City, Cleveland, Detroit, Milwaukee, and St. Louis. A ZIP code in Trenton, N.J. had the highest share of seriously underwater homes at 71 percent.

In southeastern Connecticut, the share of equity rich properties was highest in Niantic (26.7 percent) and lowest in Norwich (12.2 percent). The share of seriously underwater homes ranged from 7.3 percent in Niantic to 25 percent in Norwich.

ATTOM Data Solutions bases its home equity reports on publicly recorded mortgages and deeds of trust. The company says the reports are based on a nationwide sample of 155 million properties.

The housing data company CoreLogic also issues quarterly home equity reports, although its database looks at 50 million properties and its figures differ significantly from those of ATTOM Data Solutions. In its most recent report, for the second quarter of 2018, CoreLogic determined that about 2.2 million residences in the U.S. had negative equity – 4.3 percent of all mortgaged properties.