Relations between Maria de los Santos and the landlord of her railroad apartment in Bushwick, Brooklyn, soured after she filed a complaint in 2007 over lack of heat. But they worsened, she said, after she discovered she had been overcharged in rent and appealed to the state housing agency for help.

Ms. de los Santos said the agency did not process her complaint for 18 months and, according to the agency, she has yet to be paid the $10,000 that the landlord owes her. All the while, she said, her landlord took advantage of the delay to harass her.

“He came to the store,” said Ms. de los Santos, who runs a flower shop, “and he said, ‘If you’re still fighting for this, why don’t you just leave?’ ”

A tenant advocacy group says Ms. de los Santos’s case is not unusual. In a survey of 200 rent-regulated apartments, the group, Make the Road New York, found that landlords for nearly half the units had registered rents above what they could legally charge. Yet because enforcement is largely complaint-driven, the state housing agency usually investigated rent increases only after tenants complained.

Processing the complaints, the advocacy group found, often took at least a year.

“It’s not anything radical we’re asking for,” said Hilary Klein, a lead organizer at Make the Road New York, “just that tenants’ rights under the law be enforced.”

Landlords of rent-regulated apartments are required to report the legal rents for their units, as well as the actual amount they charge, to the housing agency, New York State Homes and Community Renewal.

Christopher Browne, deputy commissioner for policy and communications at Homes and Community Renewal, said Gov. Andrew M. Cuomo had recently overhauled rent laws.

“Enforcement is one of the things that is being strengthened,” he said.

The agency is responsible for administering laws that govern 875,000 rent-regulated apartments statewide. Last year, 1,892 cases accusing landlords of overcharges were filed, and the agency found in favor of the tenants 670 times.

Roberta Bernstein, president of the Small Property Owners of New York, a landlord association, said that while rent overcharges did happen, landlords were often hamstrung by the legal rent that they could collect, which, she said, often did not cover maintenance or routine repairs.

“I’m not denying that it occurs, but in some cases, there could be legal reasons, valid reasons, for what’s happening,” Ms. Bernstein said. “You have skewed situations where tenants are subsidized by the owner, and the owner attempts to balance the scales.”

She questioned the sources of the information in Make the Road New York’s report (see also below), which is based on rent histories of 200 households, some of which came from rent histories collected by other community organizations.

Judith Goldiner, a supervising lawyer at the Legal Aid Society, said many tenants were unaware that their apartments were rent regulated, and often too intimidated to challenge their landlords.

After Ms. de los Santos, 32, discovered the overcharge — she was paying $1,080 and it was supposed to be, according to rent records, $927 — she began paying the lesser amount.

Her landlord, Manuel Gonzalez, paid her about $3,600 for the overcharges, but Ms. De los Santos said the check did not clear.

The state agency ruled this year that Mr. Gonzalez owed her a total of $10,000, less the $3,600. He has yet to pay, and Ms. de los Santos has appealed to the state agency for help.

Reached by phone, Mr. Gonzalez initially said he did not know that his check had been returned. But he then said he was having difficulties with his properties and was short of money.

“We’re going to fix the problem with this,” he said. “We’re going to pay and we got no money, O.K.?”

Before he could be asked about Ms. de los Santos’s claim that he harassed her, he hung up.

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