Baltimore, as a matter of law, requires landlords to ensure that a home is fit for human habitation, and building officials said that includes rent-to-own landlords. But homes that are leased in rent-to-own deals can fall through the cracks because the city has so many abandoned and rundown homes.

Jason Hessler, deputy assistant commissioner for Baltimore Housing, said, “The house was in violation at the time it was sold by Fannie Mae to Vision and was supposed to be unoccupied until approved by the building department.” But he added that unless it was obvious that someone had moved into a house without the department’s permission, building inspectors might not know.

For many poor families who want to own a house and cannot get a mortgage, nontraditional housing transactions like Ms. Bennett’s have become their only option. Some do not understand what they are signing.

Dr. Lowry says that many of the families she works with do not speak English and thought they were buying a house outright. She was one of several housing officials and doctors who discussed the problems caused by seller-financed deals at a recent conference on childhood lead poisoning in Washington.

Seller-financed deals, which include contracts for deed and rent-to-own leases, are loaded with risk. They lack basic consumer protections, and residents can be easily evicted since the title to a home is not transferred until the final payment is made.

The Consumer Financial Protection Bureau has begun to investigate whether some companies are taking advantage of consumers. State regulators in Wisconsin, New Mexico and New York have begun their own inquiries, while officials in Minnesota and Missouri have issued consumer alerts.

Poor families that buy or rent one of these rundown homes often find themselves with another problem: Because they do not technically own their house, they are ineligible for any state or local grants to help defray the cost of removing lead paint.