Sellers have often been in a strong position in the real estate market in recent years. Many people have found that rising home prices have strengthened their equity share in the property, and there is a strong demand among renters and millennials to purchase a home.

However, a recent report by the national property database ATTOM Data Solutions highlights the unevenness of the market. While homeowners in several areas are experiencing high shares of equity, others are continuing to struggle with a negative equity situation.

In the second quarter of 2018, more than 5.5 million homes in the United States—10.1 percent of all mortgaged properties—were considered seriously underwater. ATTOM Data Solutions defines this situation as any property where the balance of outstanding loans secured by the property is at least 25 percent higher than the estimated market value of the residence.

"The share of seriously underwater properties has dropped well below 10 percent in bellwether housing markets such as California, Washington, Texas, Colorado, and New York, but the underwater rate remains stubbornly high in markets where price appreciation has not been as strong during the housing recovery of the last six years," said Daren Blomquist, senior vice president at ATTOM Data Solutions.

Negative equity was most persistent in Louisiana, where 21.7 percent of all mortgaged homes were considered seriously underwater. Among the 97 metropolitan statistical areas included in the report, Baton Rouge had the highest share of seriously underwater homes at 21 percent, while New Orleans was fifth highest at 18.9 percent.

Illinois had the next highest state-level share of seriously underwater properties at 18.5 percent, followed by Missouri (17.8 percent), Mississippi (16.8 percent), and Ohio (16.2 percent). Ohio had two of the five metro areas with the highest share of seriously underwater homes, with Toledo at 20 percent and Youngstown at 19.3 percent.

ATTOM Data Solutions analyzed 7,290 ZIP codes containing at least 2,500 homes with mortgages, and found 65 where more than half of these loans were seriously underwater. Eighty-one percent of mortgaged properties in Springfield, Mo. had negative equity of 25 percent or more. The next four highest shares of negative equity were in ZIP codes in the greater Chicago area.

The report determined that 12 percent of the 97,395 mortgaged homes in Connecticut were seriously underwater, along with 7.2 percent of the 16,337 mortgaged homes in Rhode Island. On a ZIP code level, 28.5 percent of mortgaged homes in Norwich and 20.6 percent of mortgaged homes in New London had negative equity of 25 percent or more. In the surrounding communities, the seriously underwater share ranged from 8.5 percent in Niantic to 13.6 percent in Groton.

On the other end of the scale, more than 13.6 million mortgaged U.S. homes—24.5 percent—was considered equity rich. In this situation, the estimated loan balances secured by the property are 50 percent or less of the home's estimated market value.

"Nationwide the number of equity rich homeowners is more than twice the number of seriously underwater homeowners, but the gap between equity haves and have-nots persists because home price appreciation is certainly not uniform across local markets or even within local markets," said Blomquist.

California continued to be the nation's hottest real estate market, with 43.5 percent of mortgaged homes in the state considered equity rich. The state also had four of the five metro areas with the highest share of equity rich homes, including San Jose (71.9 percent), San Francisco (60.8 percent), Los Angeles (47.9 percent), and San Diego (40 percent). California also had the top five ZIP codes where more than half of all mortgaged properties were equity rich.

Other states with a high share of equity rich properties included Hawaii (38.3 percent), Washington (34.5 percent), New York (33.2 percent), and Oregon (32.8 percent).

In southeastern Connecticut, 24.4 percent of mortgaged homes in Old Lyme were equity rich, along with 24.1 percent of those in Niantic and 21.6 percent of those in Mystic. Eleven percent of mortgaged homes in Norwich and 14.3 percent of those in New London were equity rich.

The ATTOM Data Solutions U.S. Home Equity & Underwater report for the second quarter of the year was based on public records and an automated valuation model. The company says its reports draw on information for more than 150 million properties in the U.S.

The property analytics company CoreLogic also issues quarterly reports on home equity, and typically sets a much lower number of seriously underwater homes. In the first quarter of 2018, CoreLogic determined that only 4.7 percent of mortgaged homes had negative equity. The company's database is smaller than that of ATTOM Data Solutions, encompassing more than 50 million homes with a mortgage.