Last summer, Nathan Connolly and his wife, Shani Mott, welcomed an appraiser into their house in Baltimore, hoping to take advantage of historically low interest rates and refinance their mortgage.

They believed that their house — improved with a new $5,000 tankless water heater and $35,000 in other renovations — was worth much more than the $450,000 that they paid for it in 2017. Home prices have been on the rise nationwide since the pandemic; in Baltimore, they have gone up 42 percent in the past five years, according to Zillow.com.

But 20/20 Valuations, a Maryland appraisal company, put the home’s value at $472,000, and in turn, loanDepot, a mortgage lender, denied the couple a refinance loan.

Dr. Connolly said he knew why: He, his wife and three children, aged 15, 12 and 9, are Black. A professor of history at Johns Hopkins University, Dr. Connolly is an expert on redlining and the legacy of white supremacy in American cities, and much of his research focuses on the role of race in the housing market.

Months after that first appraisal, the couple applied for another refinance loan, removed family photos and had a white male colleague — another Johns Hopkins professor — stand in for them. The second appraiser valued the house at $750,000.

This week, Dr. Connolly and Dr. Mott sued loanDepot, which is based in Foothill Ranch, Calif., as well as 20/20 Valuations and Shane Lanham, the owner of 20/20 Valuations. Mr. Lanham is the appraiser who conducted the first appraisal.

“We were clearly aware of appraisal discrimination,” said Dr. Connolly, 44. “But to be told in so many words that our presence and the life we’ve built in our home brings the property value down? It’s an absolute gut punch.”

The home appraisal industry, which relies partly on subjective opinions to translate home values into dollars and cents, has faced a firestorm of criticism over the past two years.