When a nonprofit developer applied to build 150 apartments for low-income seniors in the affluent Forest Hill neighborhood of San Francisco west of Twin Peaks, city housing officials were thrilled. Here at last was an affordable project proposed not for the usual precincts of the Tenderloin or SoMa, but for a quiet corner of the city that rarely sees development of any kind.

“Equity in terms of where we are creating affordable housing is a big issue for the mayor,” Jeff Buckley, senior adviser to Mayor Ed Lee on housing issues, said when the developer pitched the idea in October.

But just a month later, the project is in deep trouble — for reasons that go a long way toward explaining why most of the city’s housing for poor people is in neighborhoods where there are already plenty of poor people.

For many of those who live in the detached, single-family homes in Forest Hill, the five-story, 132,000-square-foot project proposed for Laguna Honda Boulevard is too tall and too dense. They warn that it will increase traffic, noise and crime.

On Monday night, more than 100 residents squeezed into the Forest Hill Clubhouse — a 1919 Arts and Crafts building designed by Bernard Maybeck and owned and maintained by homeowners. The occasion was a neighborhood association meeting devoted to the proposed project. The group, which typically deals with such things as pruning schedules for the elm trees along Magellan Avenue, unanimously voted against the housing development.

Joe Bravo, an attorney who is leading the charge against the project, said the opposition “is not a question of being a NIMBY. It’s a question of saving the hill” — the pine-tree-dotted knob that gives the Forest Hill neighborhood its name.

“You can’t build 132,000 square feet and make it look like the Arc de Triomphe,” Bravo said. “This is a huge development for the area. It would be perfect for somewhere along the Van Ness corridor next to a Holiday Inn.”

The project, proposed by the Christian Church Homes of Northern California, would put housing on a long, narrow parcel at 250 Laguna Honda Blvd., at the foot of the eastern side of Forest Hill. The site is now home to the Forest Hill Christian Church, a portable classroom that the church uses as a preschool, and a parking lot.

Some residents took pains to say it was the height and density of the building that they opposed, not the fact that it would house low-income seniors, 20 to 30 percent of whom would be formerly homeless. But others openly said residents of the complex would bring with them the sort of quality-of-life problems that neighborhoods west of Twin Peaks are largely immune from.

One resident said at Monday night’s meeting, “We don’t know if there are going to be sex offenders living there.” Another asked Christian Church Homes representatives, “What resources are you going to provide to make sure my 11-year-old girl is safe?”

Jamie Wong, a Sunset District native who has lived in Forest Hill for nearly a decade, said she is worried about the development destabilizing the hillside and changing the feel of the area.

“As a parent, I am concerned about people with mental illness and drug addictions,” she said. “I want my kids to be able to play outside — that’s why we bought a house here.”

The controversy underscores how antidevelopment sentiment tends to squeeze low-income projects into a limited number of dense neighborhoods. Paradoxically, it’s more expensive to build in those poorer downtown districts than in places like Forest Hill, where height limits and more expansive parcels mean projects can use cheaper, wood-frame construction. Sites near downtown tend to be small and pencil out economically only if the structures are tall, which requires costly concrete or steel-frame construction.

Kathleen Mertz, vice president of development for Christian Church Homes, said in an interview that the initial rendering for the Forest Hill project isn’t necessarily representative of what would be built. Plans could change, she said.

“I’m there to listen,” Mertz said before the meeting. “Everyone needs to participate together to help be part of the solutions of the affordability crisis in San Francisco. I would argue that being able to live and thrive and age in place in San Francisco is part of making a holistic and inclusive community. That means having housing for people at all income levels, all family types and all life stages.”

Mark Watts, president of the Forest Hill Association Board of Directors, said he supports affordable housing on the site, but that the initial proposal “looks like a Russian gulag.”

Watts added, “People are afraid of formerly homeless people wandering around the neighborhood attacking our kids and pushing our elderly down. That is not something I am worried about, but that is what people are afraid of.”

Tim Colen of the advocacy group Housing Action Coalition lives in Forest Hill and is trying to ease his neighbors’ concerns. He’s hoping to organize a tour of other developments in the city that house low-income seniors.

“People owe it to themselves to go and look at what low-income senior housing is all about, how it’s run and managed,” Colen said. “They are missing an opportunity to extend a welcome to a group of people not currently represented in the neighborhood.”

The project will ultimately need Board of Supervisors approval, which will be less likely if Supervisor Norman Yee, who represents the neighborhood, doesn’t change his mind. Given the lack of community support, he said, “I am unable to support ... this project in its current form.”

It’s been so long since affordable housing was constructed west of Twin Peaks that “it’s something new and unfamiliar” to many residents, said Kate Hartley, deputy director of the mayor’s office of housing and community.

“The fears that some people have expressed will not be realized,” she said. “We want everyone to understand that affordable housing in San Francisco is very high quality and very well done.”

J.K. Dineen is a San Francisco Chronicle staff writer. Email: jdineen@sfchronicle.com Twitter: @sfjkdineen