Supervisor-elect Dean Preston has big ambitions for San Francisco, and he hopes to shake up the status quo at City Hall when he joins the Board of Supervisors in December.

He’s been hoping to do so for nearly four years — but now his time has finally come. Preston, a tenants rights activist and democratic socialist, narrowly lost the District Five seat to then-Supervisor London Breed in 2016. This time around, he beat Supervisor Vallie Brown by fewer than 200 votes.

Preston will represent one of San Francisco’s most progressive districts, which includes the Haight-Ashbury, Inner Sunset, Hayes Valley, Fillmore and the Western Addition.

The incoming supervisor has yet to move into his second-floor office in City Hall or solidify his staff. But he’s already making plans to meet with residents and elected officials to try to jump-start his key proposals. Those plans include opening a homeless shelter in District Five, creating 10,000 units of affordable housing in 10 years, and eventually making Muni free for all.

Preston will have only a few months to prove himself before he needs to start campaigning again. Since he is only filling out former Supervisor London Breed’s term — which she vacated when she became mayor — Preston will have to run for re-election in November 2020.

The Chronicle recently sat with Preston to talk about his win and the big ideas he has for the future of San Francisco. You can hear the full conversation at sfchronicle.com/insider, and read some highlights below:

On homelessness: Preston is determined to open a Navigation Center — a temporary homeless shelter with intensive resources — somewhere in the district. He said he’s committed to announcing a site “as soon as possibe,” but did not offer a firm timeline.

Brown similarly promised to open a Navigation Center during her term, but she struggled to find a site that would work. Opening a Navigation Center, she said, was much easier said than done. Not only does a site need to be the right size and in the right location, but building one from scratch or renovating a building can also cost the city $2 million to $3 million, or more. Since the shelters are just meant to be temporary, the homelessness department says the high price tags quickly become hard to justify.

Still, Preston is adamant about opening one in the district. It is not clear what he will do differently than his predecessor, but he said he already had some sites in mind — though he wouldn’t disclose where. He said as soon as he enters office, he will meet with property owners and neighbors about where the shelter could go.

“My hope is to move forward and be able to announce it pretty quickly,” he said. “There’s overwhelming support in District Five for a Navigation Center … there’s absolutely no reason why we can get a Navigation Center in say, District Nine (the Mission), but we can’t in District Five.”

He also supports the idea of a Homelessness Commission, which would provide an extra layer of oversight to the city’s Department of Homelessness and Supportive Housing. The measure, which would have gone before voters in November, did not have the six votes it needed to move to the ballot.

Preston said he would support Supervisor Matt Haney — author of the measure — in reintroducing the commission for the upcoming November ballot. Adding his vote on the board, Preston said he’s confident it could pass.

On mental health: One of the most contentious issues in City Hall this year was whether San Francisco should expand its ability to compel severely mentally ill people into care. A state law, written by Sen. Scott Wiener, D-San Francisco, caused months of tense debate among the supervisors.

Supporters said expanded conservatorship, which is court-ordered mental health treatment, is critical for helping San Francisco curb its homelessness and mental health crisis. Opponents argued that the law infringed on personal liberties, and that the current behavioral health system didn’t have the capacity for more people.

It ended up passing with a 10-1 vote, though some supervisors were still wary of the legislation.

Preston said he would not have voted for it.

He believes the law, as written, gives too much power to law enforcement and said the number of people who would be compelled into treatment — 50 to 100 people, according to the public health department — is extremely limited, and won’t make a measurable difference on the streets.

“My view is that the concerns outweighed the benefits,” he said. “There was quite a bit of time and energy and space given to a debate that I just don’t think really struck at the heart of what the issues are.”

Instead, Preston supports comprehensive change to the city’s behavioral health care system. He was an early supporter of a sweeping plan to overhaul the system, called Mental HealthSF, which the mayor and supervisors are working together to implement.

On housing: Preston is adamant that public land should only be used for 100% affordable housing development. He said he’s not necessarily opposed to market rate development on private land — but said he would “push developers to include as much affordable (housing) in those mixed-use properties.”

He has an ambitious goal of adding 10,000 units of 100% affordable housing to the city in 10 years. This would be expensive, and likely cost the city billions of dollars and require substantial rezoning. To fund it, Preston said he will turn to the $600 million housing bond, which voters passed in November. That bond is expected to only create, renovate or preserve about 2,800 units. He said he will also consider bringing another bond to the voters in the future.

“Look, this is not cheap,” he said. “We just passed a $600 million housing bond, and that will bring significant funds into creating affordable housing, and I don’t think that’s going to be the last time that we have to do a big bond if we’re serious about getting anywhere near having a right to housing in this city.”

On transportation: Preston, who says he has been an “everyday Muni rider for 26 years,” wants to create a November 2020 ballot measure that would significantly reduce Muni fares —and make the service free by 2025.

He doesn’t have a concrete plan on how this would be funded yet, but mentioned a gross receipts tax and a transit impact development fee as two options.

“Transportation is the leading source of air pollution in urban America and in the Bay Area,” he said. “If we’re going to talk seriously about a Green New Deal and about improving both air quality and decreasing emissions in the city, we’ve got to get to a place where public transportation is fast, is reliable and is readily used, and that people are forgoing other options like Uber.”

Trisha Thadani is a San Francisco Chronicle staff writer. Email: tthadani@sfchronicle.com Twitter: @TrishaThadani