It was a moment Mischa Seligman had thought he would not live long enough to see.

On a recent Friday afternoon, Seligman, 98, walked up the steep driveway at 36 Amber Drive in San Francisco’s Diamond Heights.

Waiting for him at the top of the hill were a yellow bulldozer and two silver-plated shovels. About two dozen staff and board members of the San Francisco office of affordable housing developer Habitat for Humanity milled about. They were there to celebrate the groundbreaking of a housing complex and to honor Seligman and his wife, Brigitte, who donated the land for the development.

Seligman walked over to the construction equipment. “Bulldozers are a symbol of strength,” he said. “I love strong things.”

Strength was something Seligman had needed plenty of during his quixotic decade-long crusade to build housing for low-income families in Diamond Heights. His struggle shows how challenging it can be to build affordable housing in San Francisco, even on donated land.

This month, Habitat for Humanity will start construction on eight townhomes, at a cost of $6 million, on the woodsy hillside parcel. The units will be sold, with no money down and 0% interest loans, to households earning between 50% and 80% of area median income — $73,900 and $98,500 for a family of four. As is the case with all Habitat for Humanity projects, buyers must volunteer at least 500 hours at one of the group’s construction sites to qualify.

The units will be large — mostly three and four bedrooms — and will be able to accommodate bigger families than most affordable housing projects in the city.

“Where we now see one home, there will be eight homes,” said Maureen Sedonaen, CEO of the San Francisco chapter of Habitat for Humanity, at the groundbreaking. “And we’ll see lovely children piling out on the way to school or to catch butterflies or to do the things children do.”

For Seligman, a retired electrical engineer who lives in Santa Barbara, the groundbreaking represented the fulfillment of a dream more than 50 years in the making. His mother, the late activist Maria “Mitzi” Kolisch, who lived on the property for decades, had talked about it as far back as the 1960s. Seligman started working on it with a sense of urgency in 2011 when he turned 90 and realized that he didn’t have too many more years to make it happen.

“This has been a project of my heart, an expression of my mother’s wishes,” he said. “I was worried that I wouldn’t be around to finish it. It has been a big nuisance.”

Seligman spent years reaching out to groups that might take on the project. Initially, he hoped an affiliate of the Quakers would be a good fit, but that group eventually passed, suggesting that instead he try Conard House, which provides housing to adults with mental illness. Conard House spent more than a year analyzing the project and looking for funding, but ultimately balked at the Seligman family’s insistence on a legally binding deed restriction that would keep it permanently affordable.

Seligman then called several nonprofit developers, but they all said it was too small a project. He even wrote letters to Facebook’s Mark Zuckerberg and Craigslist founder Craig Newmark to pitch the project.

“Of course, there was no response from either of them whatsoever,” he said.

Finally, in August 2014, an exasperated Seligman called a San Francisco Chronicle reporter, who wrote an article about how he was unable to find a developer to build affordable housing on his property.

In the story, Seligman expressed his worry that he would pass away before he could find a group to build there.

“I’m in fairly good condition for my age, but I’m beginning to feel my Second World War vintage,” he said at the time.

The article caught the attention of the folks at Habitat for Humanity, who reached out to Seligman.

“If you know anything about Habitat, you know that when someone says, ‘Hey, I have a really, really obscure piece of dirt I’m looking to get rid of,’ we are going to say, ‘Yes, we will take it,’” Sedonaen said.

At the groundbreaking both Sedonaen and Seligman evoked the memory of Kolisch, an artist, political activist and medical researcher whose circle of close friends included chemist and peace activist Linus Pauling, artist Ruth Asawa, inventor Buckminster Fuller and photographer Imogen Cunningham.

Ironically, Kolisch was a stridently antidevelopment crusader back in the 1950s when the city tried to take her property as part of a large redevelopment project. Kolisch didn’t like the redevelopment because she considered it luxury housing. She “fought like a tiger” to protect her home, said Seligman, and her little cottage was one of the few structures in then-bucolic Diamond Heights that was not knocked down.

As an alternative, she drew up a plan to build homes for low-income teenagers who had aged out of the foster care system. That plan was in its early stages when she died in 1987.

Seligman said his mother would have been thrilled with how things turned out — even if it did take more than 30 years to get it done.

“She never liked for things to go to waste,” Seligman said. “She would say, ‘If I have enough to eat, I don’t need more. Let someone else have it.’ Unfortunately a lot of people these days have this need to amass more and more money even if they don’t need it.”

Already Habitat’s workers and volunteers have started clearing the site for development. A few weeks ago they cut down a row of giant eucalyptus trees Seligman planted more than 50 years ago from saplings he carried there in coffee tins.

Construction will take about a year and a half. The eight families will settle into their new homes in 2021 — about the time Seligman turns 100. Asked if he will be around to visit them, he nodded.

“Of course,” he said. “Why wouldn’t I?”

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