San Francisco’s homelessness director wants them. A local developer is hot to build them, academics love them, and unions are open to the idea.

All that stands in the way of the construction of hundreds of tiny, modular apartments for hard-core homeless people in San Francisco — something that could sharply reduce the number of indigents on the city’s streets — is getting everyone involved to agree to some compromises. But that’s proving to be a tough task.

For the past year, developer Patrick Kennedy has been pitching City Hall on his plan to use metal shipping container-style boxes in quickie-construction projects that can be turned into supportive housing for the homeless — complexes that offer services for people in addition to a roof. Such housing, Kennedy says, can be built far faster than conventional structures, for half the cost.

Kennedy has proposed building as many as 200 units above a city-owned parking lot at Highway 101 and Cesar Chavez Street, and he says he can quickly construct thousands more around the city.

Separately, Community Housing Partnership, one of the biggest supportive-housing developers in the city, is exploring building a 100-unit complex of modular units made of wood, which would also be cheaper and quicker than conventional construction.

In its June series on possible solutions to the city’s homelessness problem, The Chronicle explored the urgent need to add to San Francisco’s stock of 6,000 supportive housing units and showed how building modular units could be a useful technique.

Mayor Ed Lee has pledged to spend $1 billion over his second term to house 8,000 homeless people. He said earlier this year that he liked the concept of modular housing, but that details had to be worked out by city staff — and that’s where the hurdles have appeared.

The main construction unions that would be involved are balking at the fact that Kennedy’s metal boxes would be built in China, not by American union workers, and say they believe building-code requirements are less stringent for modular construction. And Jeff Kositsky, head of Lee’s newly created Department of Homelessness and Supportive Housing, is resistant to Kennedy’s proposal to build on city-owned land, saying there are already too many demands being made on scarce public property.

Kennedy, owner of Panoramic Interests, says those concerns can be addressed. He’s going to begin displaying a full model of one of his microunits in front of his office at Ninth and Mission streets on Oct. 21, and he hopes the reality version of his vision will kick the discussions into higher gear.

“We’ve been kind of holding on back pushing the subject further until we have something concrete for people to come see and experience,” he said. “But after we get this unit in front of my office, I want to talk more.”

Gail Gilman, head of Community Housing Partnership, said her wooden units would be built in Idaho, avoiding the import debate. But one disadvantage of wooden units is that they are more costly and time-consuming to put together than the metal project Kennedy has in mind.

Gilman is exploring sites and says she’s eager to work out details with the unions and the city.

“We should really have the city rally and come together to say, ‘Let’s try a couple of modular projects and see how it goes’ — because homelessness is a crisis here, and we need to try things,” Gilman said.

Kositsky said that if details can be worked out over such matters as location and the union issues, he could be on board.

There are at least 6,700 homeless people in San Francisco, according to the city’s 2015 point-in-time count. The city is under increasing pressure to house them, particularly the estimated 1,500 chronically indigent who are the most troubled — those erecting tent colonies, using drugs in the open and requiring about $80,000 per person each year in emergency services.

Hard-core street people need not just housing, but apartments with counseling — the “support” in supportive housing — in the same building to help them get over their addictions or other dysfunctions. Kositsky’s challenge is finding a way to put roofs over people in one of the most expensive housing markets in the country.

Only a few modular housing complexes for the homeless have been built in the United States, and all of those are in New York and Los Angeles. Those are wood-framed — no such housing has been constructed out of metal, as Kennedy hopes to do. The technique is often called “Lego housing” because of the way the units can stack.

“I love the idea,” Kositsky said. “The city is interested in finding ways to lower construction costs and the length of time it takes to build, and modular housing is certainly a promising development — and not just for homeless housing, but all housing.”

He said modular construction is not by definition “low-quality housing — there is plenty, used for hotels and dorms and more, that has been high-quality.” The luxury, metal-modular citizenM Hotel in Amsterdam is an often-cited example of well-done modular construction, as is the 2-year-old, wooden Star Apartments supportive housing complex in Los Angeles.

Kennedy’s proposal for 200 units atop a parking lot at Highway 101 and Cesar Chavez Street would leave the parking intact while bolting the apartments together above it. If the city won’t lease him the land, Kennedy said, he’ll look for a privately owned site to purchase, though that would increase the cost.

His plan is to pay for construction himself, then lease the complex to the city for $1,000 a month per unit. Working with private land instead of public land would raise the lease cost to about $1,200 a unit.

Kennedy and city officials agree that either option would be cheaper than many alternatives for constructing housing for the homeless. Conventional affordable housing costs about $400,000 per apartment to build. Kennedy says his Lego units would cost about $200,000 each, and that he would absorb the up-front expense.

And they’re a fast build. Kennedy says he could complete construction in nine months, compared with as long as five years for more conventional buildings.

The units are tiny, at 160 square feet, but that’s about the same size as many of the renovated residential hotel rooms that the city leases for homeless people. According to Kennedy’s architectural plans, once they’ve been finished with drywall, glass and paint, they will look no different from typical construction.

Michael Theriault, secretary-treasurer of the San Francisco Building and Construction Trades Council, said his unions “are not fundamentally opposed to modular housing. But we’d rather they be constructed here instead of China so they don’t undercut wages and conditions. And we want them built under local building code and inspected by local inspectors.”

Unions worked with Kennedy three years ago to build a small, market-rate complex of larger wooden modular units south of Market Street. And last year, unions worked with developer Rick Holliday to erect a 136-unit apartment complex in the Bayview with wooden modulars. Units for both projects were made at the same now-defunct factory in Sacramento.

“We should be able to work things out,” Kennedy said.

Kositsky said, “I’m willing to sit down and talk if it meets code, provides good jobs, and if he’s not talking about using public land — that land is precious, and we have to be good stewards of it. We’re already master leasing from private landlords (of residential hotels). Why not this, too?”

Carol Galante, a UC Berkeley professor who specializes in affordable housing and urban policy, said it’s worth working out differences because modular housing shows great promise.

“Modular construction saves time and money — at least 20 percent in construction costs and 40 percent in time — that’s been proven,” Galante said. “We can do this.”

Down in Los Angeles County, where 57,000 people live on the streets, Kennedy’s microunit idea is getting a smoother reception.

Marc Trotz, director of housing for the homeless for the Los Angeles Department of Health Services, wants to create 10,000 units of supportive housing over the next few years — and he has had serious talks with Kennedy and others about contracting for complexes of metal units.

“I am totally jazzed about what Patrick is doing,” said Trotz, who created some of San Francisco’s most successful supportive housing before going to Los Angeles several years ago. “There has to be something between a cardboard box on the street and a $400,000 unit that we can do. We have to start innovating, and modular housing is very exciting.”

About 30 miles south of Los Angeles, the nation’s first supportive housing complex constructed of metal stacking units will begin being assembled this week in Midway City (Orange County).

American Family Housing’s 16 apartments for homeless veterans each consist of three metal boxes bolted together, not just one box per apartment as in Kennedy’s plan, but they are the same concept. The kicker? They are actual metal shipping containers, made in China and sold secondhand as surplus over here.

Begun in May, the project is penciled in to take nine months to build, at half the cost of regular construction.

American Family Housing “is trying to innovate and change the way permanent supportive housing is done,” said company President Donna Gallup. “If the rest of the world did housing this fast, we’d be able to end homelessness a heck of a lot quicker.”

Kevin Fagan is a San Francisco Chronicle staff writer. Email: kfagan@sfchronicle.com Twitter: @KevinChron