In a move that could result in Silicon Valley billionaire Vinod Khosla being hit with tens of millions of dollars in penalties, the California Coastal Commission has begun an enforcement action against Khosla for his refusal to open Martins Beach in San Mateo County to the public.

The commission last Thursday sent Khosla, a venture capitalist and co-founder of Sun Microsystems, a letter charging that he has committed “numerous and significant violations of the California Coastal Act” stretching back to 2009, a year after he purchased a beachfront property south of Half Moon Bay and locked the gate to a beach that had been enjoyed by families since the 1920s.

Because Martins Beach is flanked on both sides by tall cliffs, the road is the only way for the public to get to the beach.

Under a law signed by Gov. Jerry Brown in 2014, the Coastal Commission can fine people who build without permits along the coast or who block public access to beaches up to $11,250 per day — or $4.1 million per year — up to five years.

The letter the commission sent to Khosla names at least five potential violations, including grading and subdividing the property without a permit, and potentially several more that could bring the penalties to more than $20 million. He also could face additional penalties of $15,000 a day — or $5.47 million a year — for up to three years from a court — and could have liens placed on the property if he refuses to pay.

The commission’s letter, signed by Executive Director Jack Ainsworth, gives Khosla until Oct. 4 to respond and says that agency remains “willing and ready” to meet with him to negotiate a compromise to open the gate. Barring that, a decision to issue a “cease-and-desist order” with penalties could be made as soon as the commission’s next board meeting in November in Bodega Bay.

“Martins Beach is a really popular beach,” said Lisa Haage, chief of enforcement at the Coastal Commission. “It’s historically been a family resource for people from all over California for fishing, picnics and cultural activities. It has been a gathering spot for generations.

“As the population grows, there is no more coast. Every single access point is important, but this one in particular is a real rallying point for a large number of people.”

When asked Tuesday for reaction to the commission’s decision, Khosla’s attorney, Dori Yob of San Jose, referred a reporter to a blog post she wrote in April 2016 that said Khosla views the issue as a private property rights matter that he will continue to fight out in court.

“Only the courts can decide what the proper interpretation of the law here is, and we intend to let that process determine what is right and what is wrong,” Yob wrote. “No media vilification campaigns or politically motivated opportunistic proclamations or state bills by pandering politicians can decide the issues at play.”

Advocates for public access for fishermen, surfers and other beachgoers said Tuesday they are pleased at the commission’s crackdown — and that the case sets an important precedent.

“I’m gratified that our institutions are doing what they were put there for — and not being bullied into letting this one slide,” said Mike Wallace, an economist in Moss Beach and coach of the surf team at Half Moon Bay High School. “This is a litmus test for a cascade of similar events up and down the coast if it wasn’t enforced. I applaud the commission for doing this.”

In December, the commission issued a $4.2 million fine against Dr. Warren M. Lent and his wife, Henny, for refusing its requests to remove a fence, gate, stairway and deck on their property that was blocking a public pathway to a beach in Malibu. Commissioners, who had spent nine years on the issue, called it “very flagrant” and an “egregious violation” of state law in an area known for wealthy residents attempting to block public access to the shoreline. The couple has since filed a lawsuit against the commission, something Khosla did last year.

In the Khosla case, Haage said, the commission, which first sent Khosla a letter in 2011 saying he was violating state law by closing the private road to the beach, waited to move forward with enforcement until it could see how a key lawsuit played out.

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That lawsuit, filed by an environmental group, the Surfrider Foundation, claimed that Khosla was violating the Coastal Act, one of California’s landmark environmental laws, because he did not get a permit from the commission to change the public access levels at the beach or for the gate. A lower court agreed, and last month a state appeals court also ruled against Khosla and required him to open the gate immediately.

On Monday, Khosla appealed that decision to the state Supreme Court.

Meanwhile, a bill that would ease the state’s ability to take a public easement over the road by eminent domain cleared the Legislature and is now on Gov. Jerry Brown’s desk. That bill, SB 42, by Sen. Jerry Hill, D-San Mateo, would set up a State Lands Commission fund that could accept private donations to pay for the easement, which the commission appraised at $360,000. Khosla had told that commission he would only consider selling the right for the public to use the road for $30 million, almost what he paid for the entire 89-acre property in 2008.

Because of the appellate court ruling, the San Mateo County Sheriff’s Office has told the public to feel free to ignore Khosla’s no-trespassing signs.

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