An increasing number of homeowners have a hefty amount of wealth locked up in their property's equity, according to the real estate data company ATTOM Data Solutions.

In its U.S. Home Equity & Underwater Report for the fourth quarter of 2018, the company determined that 14.5 million mortgaged properties—or 25.6 percent of all residences with open loans—were equity rich. ATTOM Data Solutions defines an equity rich home as one where the combined balance of the loans secured by the property is 50 percent or less of its estimated market value.

This total marked a year-over-year increase of 834,000 properties, reaching the highest number since the company began tracking equity rich data in the fourth quarter of 2013. The share of equity rich homes was down from 25.7 percent in the third quarter, but up from 25.4 percent in the fourth quarter of 2017.

ATTOM Data Solutions said the rising share of equity rich properties was driven in part by lengthening homeownership tenure, with people staying in their homes for longer periods of time and gradually building up equity through regular monthly payments. The report also noted how skyrocketing prices in the West have contributed to stronger equity in this region, while negative equity is more persistent in the South and Midwest.

"With homeowners staying put longer, homeownership equity will most likely continue to strengthen," said Todd Teta, chief product officer at ATTOM Data Solutions. "Those that are seriously underwater may find themselves coming up for air as they continue to pay off excessive legacy mortgages or sell."

California had the highest share of equity-rich properties at 43.6 percent. Of the 98 metropolitan statistical areas included in the report, those with the highest share of equity rich properties included San Jose (72 percent), San Francisco (60.7 percent), and Los Angeles (48.5 percent). An analysis of 1,479 counties with at least 2,500 mortgaged properties found that seven of the 10 counties with the highest share of equity rich shares were in California, while the top five ZIP codes where more than half of the properties were equity rich were also located in the state.

Hawaii had the next highest equity rich share at 39.3 percent. This was followed by New York, Washington (both 34.2 percent), and Oregon (32.9 percent).

At the local level in southeastern Connecticut, equity rich levels ranged from 11.5 percent in Norwich to 25.4 percent in East Lyme.

The report also determined that more than 5 million homes, representing 8.8 percent of mortgaged properties, were seriously underwater. This share was unchanged from the previous quarter but down from 9.3 percent in the fourth quarter of 2017. A home is considered to be seriously underwater if the combined value of the loans secured by the property is 125 percent or more of its estimated market value.

More than one in five mortgaged properties in Louisiana—20.8 percent—were considered seriously underwater. Baton Rogue had the highest share of seriously underwater homes among the metro areas included in the report at 20.7 percent, while New Orleans was third highest at 19 percent.

Other states with a high share of seriously underwater homes included Mississippi (16.9 percent), Arkansas (15.9 percent), Illinois (15.6 percent), and Iowa (15.2 percent). In Ohio, the seriously underwater share stood at 19 percent in Youngstown and 18 percent in Toledo.

At the ZIP code level, more than half of the mortgaged properties in 27 communities were considered seriously underwater. These included areas in the metro areas for Atlantic City, Chicago, Cleveland, Detroit, St. Louis, and Virginia Beach.

ATTOM Data Solutions' equity reports are based on information from a nationwide database of more than 155 million properties. The results typically vary considerably from similar equity reports issued by CoreLogic, which looks at a database of more than 50 million homes.

CoreLogic has not yet issued an equity report for the fourth quarter of 2018. Its third quarter report determined that 2.2 million homes, or 4.1 percent of all mortgaged properties in its analysis, had negative equity.