The woman on the first floor hung a sheer white curtain across the middle of the room, to divide the space with the bed from the space with the table and chairs.

The room where she and her husband live is about 23 feet long and 11 feet wide. They pay $650 a month, plus electricity. Their room has been illegally converted into living quarters in a three-story apartment building in Bushwick, Brooklyn.

According to city records, the building's certificate of occupancy allows for five families in five units. But with rooms in the basement and other floors rented out, 9 to 12 families live there. The woman, Guadalupe, 49, asked that her last name and address not be published, for fear of being forced out of the place she and her husband, both immigrants from Veracruz, Mexico, have called home since November.

The room in Bushwick is one of thousands just like it in immigrant neighborhoods in Brooklyn, Manhattan, Queens and the Bronx. These rooms form a secret world of New York City housing: hidden from public view, unregulated and, too often, unsafe.

Earlier this month, a trial in the Bronx offered a brief glimpse into that world.

Two tenants of a Bronx building were accused of erecting drywall partitions in their apartments to rent out windowless rooms for $75 to $100 a week. When a fire broke out one Sunday in 2005, two firefighters died after jumping from a window, and prosecutors argued that the illegal partitions left the firefighters disoriented and forced them to jump to their deaths. One jury acquitted the tenants, and a separate jury found the building's owner and former owner guilty of criminally negligent homicide.

Illegally converted rooms have long been a fact of life in some of the city's poorest communities.

Some rooms, like those in the Bronx case, are carved up by tenants hoping to turn their apartments into a source of income. Others, in Chinatown in Manhattan and Jackson Heights in Queens, were converted by building owners to generate more rent. All are constructed without regard for the city's building and housing maintenance codes, which govern maximum permitted occupancy, means of exit, ventilation and lighting.

In their room in Bushwick, Guadalupe and her husband, Pedro, have no stove, just a microwave oven on top of a big television. They keep their clothes and shoes on a metal rack. They have no lease and no mailbox. They give the money for the rent and electricity to a first-floor tenant with a lease, who slips their mail underneath their door.

For all of this, Guadalupe said she feels lucky. The couple have their own bathroom. In a nearby building where they used to rent a room, they often had to wait in line to use the toilet.

"Have you ever been to the Lower East Side Tenement Museum?" asked Javier Valdes, who visited Guadalupe's room recently and who is deputy director of Make the Road New York, a community services group of which Guadalupe is a member. "It's like a repeat of history. It's just a different group of people going through it."

Throughout the city illegal units make for ad-hoc living arrangements, and are not always done for financial reasons. Last week, the superintendent of a building in the Bronx owned by the nonprofit Fordham Bedford Housing Corporation saw a tenant carrying construction materials into an apartment. The tenant in the one-bedroom apartment had split the living room in half by erecting a wallboard partition, to make two bedrooms for family members who had moved in.

"What goes on behind tenants' doors, we don't always know," said Robert Donovan, the group's director of construction services, who told the tenant to take the wall down.

Often, little construction is required. Tenants or landlords simply put locks on bedroom doors to rent them out as rooms. Residents of illegal units sometimes pay not for rooms but for the use of mattresses in those rooms, in a practice known as "hot sheeting." It means several people share a bed at different times of the day.

"We have heard of those cases," said Mr. Valdes of Make the Road New York.

A firefighter in East Harlem recalled the day several years ago when he found himself in an apartment crowded with plywood bunk beds where men were sleeping in rotating eight-hour shifts, "like on a submarine," he said.

A report released last year that analyzed census data from 1990 and 2000 found 114,000 "unaccounted-for" units in New York City, housing 300,000 to 500,000 residents. These illegal units included private homes carved into rooming houses and two-family homes with unauthorized basement apartments.

Researchers with the two groups that prepared the report, the Pratt Center for Community Development and Chhaya Community Development Corporation, said the 114,000 units made up roughly 4 percent of the city's housing stock. A majority were in Queens, Brooklyn and the Bronx, with Queens having the most of any borough with 48,000.

The report described many of the units as reasonable options for low-income tenants struggling to find affordable places to live, a view shared by many housing advocates. The two groups called for the city to legalize many of the units and to offer landlords technical and financial assistance to make needed repairs.

"We really have effectively a 'Don't ask, don't tell' system," said Brad Lander, director of the Pratt Center. "The only remedy is to vacate the unit, and that's what we're looking for a way out of."

Another group, Coalition for the Homeless, has criticized the city for helping to fill dozens of illegal and overcrowded boarding houses. The coalition has identified more than 100 unregulated rooming houses where the city's homeless services agency and its contract shelters have placed homeless adults, even though many buildings were found to have hazardous conditions.

For Guadalupe, the $650 rent is getting hard to pay. She makes and sells tamales, and Pedro works in construction. They had to borrow $200 from cousins to pay this month's rent.

Their time in the room is nearing an end. Because the tenant to whom she pays rent is moving out soon, she said he told her that she must go, too. She bought a receipt pad and makes him fill it out when she pays the rent, one small way she tries to impose some order on a precarious situation.

"On any given day," she said, "I feel I can get kicked out."