She was a glamorous, successful advertising executive who seemed to be leading the idyllic Manhattan life in a posh Upper West Side flat. But Susan Trott’s murder in her apartment last Sunday revealed she was an outcast in her own building long before the killer walked through her door.

The 70-year-old woman was found with her throat slashed in her West End Avenue co-op after cops got a call from her business partner who had not heard from her in days.

The next day, police officials announced that they had a suspect — a woman who lived in the building. The neighbor is now at a hospital for a psychiatric evaluation. She has not been publicly identified and has not been charged with a crime.

A motive remains unclear, but detectives are zeroing in on the 16-story prewar brick building at 710 West End Ave., where one-bedroom apartments go for nearly $1 million.

“We believe the answers are within that building,” NYPD Chief of Detectives Dermot Shea said last week.

From the outside, the building between 94th and 95th streets doesn’t stand out from its neighbors on the quiet stretch, about a block from Riverside Park.

But for Trott, the co-op could be a living hell.

For more than a decade, she fought neighbors who attacked her over her apartment, her beloved dogs and her habit of feeding birds outside, her longtime business partner and friend, Eric Boscia, told The Post.

On occasion, said Boscia, these battles escalated into physical altercations. He wonders whether the hostility led to her murder.

“I said it would be someone in the building,” he told The Post. “I said if anything looks foul, it was someone in the building.”

Trott was a “quintessential New Yorker” with a passion for art and fashion, said Judy Segaloff, another longtime friend and business associate.

“She was like a character out of ‘Sex and the City,’ ” Segaloff said. “She would walk into the stores on 57th Street and Fifth Avenue, and she would basically know all the salespeople and look at the latest Gaultier or the latest de la Renta.”

The Queens-born Trott moved into the apartment in the 1980s, after splitting from her second husband, Paul Wolfe. By then, she was a successful advertising copywriter and consultant, working on notable ad campaigns in London and New York.

“The word is ‘brilliant,’ ” Segaloff said. “She’s just one of these people that was just brimming with creativity.”

By the end of her life, she counted household names among her clients, including Ambien, Virgin Atlantic and Air Canada, all while running her own ad company out of her apartment, with Boscia based in London.

“She was doing the work of a Madison Avenue advertising agency right out of that apartment,” Segaloff said.

It was in the early 2000s, after more than a decade of living at the address, that Trott’s troubles in the building began.

The Upper West Side neighborhood had transformed. Creative types like Trott who had populated the area were being replaced by doctors, lawyers and bankers. Trott got the sense that her co-op board was trying to force her out to make room for the new set, she told friends.

The first fight began when Trott bought a vacant apartment next to her own, Boscia said. Trott would have visiting friends stay in the next-door flat, causing some neighbors to suspect she was illegally renting it. Their complaints ignited the board’s fury.

“She would say, ‘They’re my friends, they’re from Australia, they’re from Japan, they’re staying for like three to four weeks,’ ” Boscia said. “[The board] were giving her issues with that.”

Eventually, the board made her knock down the wall separating the apartments, he said.

Next, there was a dust-up over her American bulldogs, Tulip and Mr. Pink. Trott, who volunteered throughout her life for several animal-welfare charities, had rescued the dogs when both were near death, Boscia said.

“She was taking them out in the middle of the night to go to the bathroom,” Segaloff said. “And when they couldn’t walk anymore because they were so sick, she would carry them.”

But her neighbors accused her of leaving the dogs off their leashes and complained that they were aggressive toward other dogs in the building.

After years of arguing, the board banned large dogs from the building, Trott told friends.

Then there was a controversy over her feeding birds. Trott would carry a loaf of bread or a bag of birdseed with her to feed pigeons on West End Avenue and in Riverside Park.

Residents said the birdseed attracted rats. But some who knew her from outside the building saw it as a good-hearted habit.

“She was a very prominent person, who in retirement was going around the neighborhood, feeding the birds and the squirrels,” said Stefan, a local retiree who did not want to give his last name.

“That was her passion. She would go to Riverside Park. She would go to Joan of Arc Park. She’d go in the middle of Broadway. She’d walk up and down, feeding God’s creatures. She’d have a bag full. She took care of God’s creatures. All the birds know her.”

Stefan conceded it probably drew vermin but added, “Whether she did it or not, there would be an infestation. They’d still eat. You see rats stealing whole slices of pizza!

“She was just trying to do good in her later days. Whenever she was in my presence, she was very respectable. I’ve seen her do nothing but good.”

Building residents approached by The Post would not comment, but the president of a local neighborhood association confirmed there was friction.

“She was mostly not liked,” said Aaron Biller, who runs the Neighborhood in the 90s association.

“A lot of people didn’t like her because she kind of industrialized feeding the birds,” Biller, 64, said. “It was feeding the birds, but it was also feeding the rat population. And that was a source of contention throughout the neighborhood.”

Through it all, the feisty divorcee fought back.

“She just stuck it to them, like, ‘I’m not leaving,’ ” Boscia said.

Trott asked other doorman buildings in the neighborhood about their dog policies so she could show her board was acting unfairly. And she snapped photos of the large trash piles on the street outside the building in order to argue that they might be attracting rats to the block.

As the battles wore on, the tensions in the building escalated, Boscia said.

“They were writing anonymous letters to the board about her — that would go on for years, making up stories about her,” he said.

On one occasion, her clothes were strewn around a basement laundry room after she left them in a machine.

“She had neighbors harassing her,” Boscia recalled. “I’ve witnessed it. They’d be outside yelling at her for things, for feeding the birds. They would say something to her about the dog, like, ‘Get your dog back!’

“And I think it got very serious this summer,” Boscia added. “A lot of times, she wouldn’t want to leave the apartment.”

The NYPD said on Saturday that Trott had filed no complaints against any neighbors and that no complaints were made against her.

But for Boscia, it’s no shock the suspect shared her address.

“The people that abused her most were her neighbors,” he said.

Additional reporting by Khristina Narizhnaya