When Patricia O’Grady moved into the top floor of a Greenwich Village walk-up in 1955, she and her three roommates helped sweep the hallway in exchange for a discounted rent of $16 a month.

The unit was bare, no more than floor and walls, so the girls, all aspiring actresses, slowly improved it themselves, installing a sink and other modest amenities. While her roommates moved on, O’Grady never left the unit, and for that she received the ultimate New York City prize: unbelievably affordable rent.

Until March, when O’Grady, 84, was fatally struck by a car just a few feet from her home, she paid $28.43 a month for the apartment.

“I consulted with an attorney to find out if this rent was possible,” recalls Adam Pomerantz, who bought O’Grady’s building, which also houses his business, Murray’s Bagels, in 2002.

Google Street View shows the street on which the apartment is located, with Murray's Bagels at 500 Sixth Ave. Google Maps

It was legit, he found, but using a rent-control-formula worksheet, he was able to increase her rent a whopping $1.98 — it had previously been $26.45.

O’Grady was always early with her payments, Pomerantz says, in part because, despite the fact that his tenants can hand in their rent downstairs at the bagel shop, O’Grady insisted on mailing her check.

“She was the only one who walked to the corner to mail it,” says Pomerantz. “She was very quick paying her $28.”

In addition to possibly being the cheapest unit in Greenwich Village, the apartment may also be New York City’s last cold-water flat: It had neither heat nor hot water. There were, however, two working fireplaces.

O’Grady was so set in her ways that she fought Pomerantz any time he tried to update, or even make necessary fixes, such as putting floorboards over gaping holes.

When Pomerantz attempted to install proper heat, he recalls, she pleaded with him: “What you’re doing to me is torturing me. Please leave the apartment as is. I’m at peace.”

“She didn’t want it. she didn’t want anything,” he says.

The apartment had no bath or shower. There was only a single gas lightbulb. Due to her osteoporosis, O’Grady struggled to replace it when it went out, preferring to then live by candlelight. The pull-chain toilet and cast-iron stove were updated only recently.

“I didn’t even know that [cold-water flats] existed anymore,” Ava Farkas, executive director of the Metropolitan Council on Housing, said in a phone interview. “I think that’s highly, highly rare.” Farkas had not heard of a lower current rent in New York City.

O’Grady spent her days at the 14th Street YMCA, where she swam, showered and read the New York Times, friends remember. Her sister, Roberta, who still lives in their hometown of Oakland, Calif., shared the unit with Patricia in the early 1960s when Roberta was pursuing a master’s degree at New York University.

“We would heat the water on the stove and then pour it into the old-fashioned washtubs against the wall,” Roberta said in a phone interview, going on to say of her sister, “She was very tough-minded” — a universal refrain among those who knew her. “She wanted to pursue her acting career, and this apartment made it possible.”

“ Even studio apartments in the neighborhood rent for $3,000 to $5,000 or even more, according to current Realtor.com listings. ”

Sticking it out in the unit was no easy feat. In addition to the lack of most modern conveniences, past landlords also tried to force O’Grady out. “A fire was set at some point,” with the intention of evicting rent-controlled tenants, says Roberta, “Everybody else left except her.”

The building’s only other current residential tenant, 33-year-old Steven Flisler, a producer at NBC, became close with his neighbor.

“I came back many nights and it’d be like 7 or 8 o’clock, or sometimes 1 or 2 in the morning, and she’d be sitting on the stoop, reading her mail, and I’d always spend 15 to 20 minutes talking with her about history and current events,” he says.

“I work in news, but I’d find out tidbits from her about what’s going on — she’d read the newspaper cover to cover.”

Despite her hunched back, O’Grady continued to attend dance classes twice weekly at the Joffrey Ballet School, where she was a longtime and beloved student.

“She took classes at the school for at least 30 years,” says Stephanie Godino, a teacher at Joffrey. The school held a small memorial service for O’Grady when she died.

The Joffrey School is just over two blocks from O’Grady’s home, and she rarely ventured much farther, at least when she was in the city. “I feel like she never went past 14th Street,” says Flisler.

She did, however, cross the country by train once a year to visit Roberta in Oakland. Before leaving for her trips, “She’d always write me a letter that was like, ‘I’m going away, hope everything’s well, I’ll be back in two weeks’ — that old-school tradition,” Flisler smiles.

In turn, Roberta would spend every Christmas at Patricia’s until the early 2000s, when Roberta began going to the nearby, now-closed Larchmont Hotel.

“I was getting too old to bathe that way,” Roberta says.

Many in the local theater community reached out with fond memories of Patricia, who was something of a legend in her little corner of New York.

“I was lucky enough to rehearse a couple of scenes with Pat in her apartment back in the ’80s when we were in class together,” recalls Edith Meeks, the executive and artistic director of HB Studio. “I remember a Siamese cat and a wood-burning stove, racks of costumes and a living room that she used as a rehearsal space.”

According to her obituary, which, Roberta says, Patricia wrote some five years before her death, she had credits in the 1976 films “Next Stop, Greenwich Village” and “Taxi Driver.” The obituary quotes director Herbert Berghoff, who directed a number of the more than 20 productions O’Grady appeared in at the HB Playwrights Foundation, as referring to her as one of the best actresses in America.

To the end, it seems, no one but O’Grady and Pomerantz knew her exact rent — “I thought it was $35,” Roberta says. Even Flisler was surprised when he heard the amount, “I knew it was a couple bucks less than mine,” he laughs, admitting he hadn’t realized quite how many bucks less.

“She told us $50. I guess she didn’t want us to know,” Joffrey School instructor Liz D’Anna says.

“A splendid woman,” Meeks says of O’Grady, “an artist, one of those singular people that New York seems to have a unique capacity to shelter.”

With O’Grady gone, Pomerantz will be gutting and renovating the apartment, renting it out as a two-bedroom in the $5,000 range.