Sen. Dianne Feinstein is coming down hard on the side of Presidio Terrace residents outraged that San Francisco sold their gated, private street to a South Bay couple.

Feinstein, who lived in Presidio Terrace for more than 20 years, wants the Board of Supervisors to take the unprecedented step of overturning a tax-default sale that delivered the neighborhood’s common areas to the young couple. It was bureaucratic bungling by the city’s treasurer-tax collector’s office, she said, that resulted in residents’ failure to pay property taxes on the street and sidewalks for three decades.

The state’s senior senator told the supervisors in a letter mailed Monday that her first reaction to the news that the street had been sold for $90,100 was one of “near disbelief.”

“I would not have guessed bureaucracies still held surprises for me,” the former mayor wrote. “But this one did.”

In the letter, Feinstein said there were “serious principles at stake” in the decision by the treasurer-tax collector to seize and sell the block-long, circular street and its sidewalks after the homeowners association failed to pay the street’s $14-a-year property tax for 30 years. With penalties and interest, the total owed by the residents of the multimillion-dollar homes was $994.

The street was scooped up in an online auction in 2015 by Tina Lam and Michael Cheng of San Jose. The first the homeowners heard about it was when a representative of the married couple approached them about their interest in buying back the property earlier this year — well after the statute of limitations for challenging the tax sale had passed.

The 35 homeowners took their case to the supervisors, who agreed to hear their request to undo the sale.

Homeowners say they weren’t being deadbeats. They blame the treasurer-tax collector for sending tax bills to a former bookkeeper for the association who had retired in the early 1980s, and then sending the tax-sale notice to the same incorrect address.

Feinstein agreed, telling the supervisors, “For decades, those bills were presumably ‘returned undeliverable’ to the same office.

“Yet continually informed by the apparently decades-long feedback loop that the wrong address was still wrong, the office’s ignorance endured,” Feinstein wrote. She noted that there was “no notice whatsoever” posted on the street when the city scheduled its auction sale.

The senator said the alleged failure to notify residents of the sale amounted to an unconstitutional lack of due process.

Feinstein said the insistence of the treasurer-tax collector’s office on “wholly blaming” the homeowners for the fiasco “was breathtaking.”

Treasurer Jose Cisneros, however, said his office followed all the right procedures.

“I am disappointed the senator feels we acted ignorantly,” Cisneros said Tuesday.

“Every year more than 10,000 bills and notices are returned because of an incorrect or out-of-state address, and most of those homeowners still manage to pay their property taxes,” he said.

“The law is clear,” Cisneros added. “Whether or not an owner receives a bill, taxes are due.”

While the homeowners can count Feinstein as an ally, the street’s new owners are arming up as well for the Nov. 28 supervisors hearing. They’ve hired Shepard Kopp, the attorney who persuaded supervisors to rescind Mayor Ed Lee’s firing of then-Sheriff Ross Mirkarimi in 2012 after he pleaded guilty to a misdemeanor in a domestic violence incident with his wife.

“In a nutshell, the board’s role here is to determine if the tax collector followed the law,” Kopp said. “If the board finds that the tax collector did follow the law, then that should be the end of the inquiry.”

Kopp said county supervisors across the state rescind tax sales only when there has been a mistake, “and there was no mistake here.”

Interesting to note that Kopp is the son of longtime Bay Area politico Quentin Kopp, who was among Feinstein’s biggest nemeses during her time at City Hall.

Far from being ruffled by the senator’s entrance into the debate, Shepard Kopp called Feinstein’s letter “fantastic” — saying it “plays right into my narrative that this is government by and for the rich.”

“It leads one to wonder if Feinstein would be putting in such an opinion letter if it affected anyone other than her former friends and neighbors,” Kopp said.

In her letter, Feinstein acknowledged that “my own family’s time as Presidio Terrace residents” lends a “personal perspective” to the issue. But she argued that it’s irrelevant to the “underlying principles that should be decisive to a just resolution.”

“In the United States, no one should lose property at the hands of the government without knowing it,” Feinstein wrote.

San Francisco Chronicle columnists Phillip Matier and Andrew Ross appear Sundays, Mondays and Wednesdays. Matier can be seen on the KPIX TV morning and evening news. He can also be heard on KCBS radio Monday through Friday at 7:50 a.m. and 5:50 p.m. Got a tip? Call (415) 777-8815, or email matierandross@sfchronicle.com. Twitter: @matierandross