A property owner who illegally demolished a 1936 Twin Peaks house designed by a renowned modernist must rebuild an exact replica of the home rather than the much larger structure the property owner had proposed replacing it with, the City Planning Commission ruled this week.

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In a unanimous 5-0 vote late Thursday night, the commission also ordered that the property owner — Ross Johnston, through his 49 Hopkins LLC — include a sidewalk plaque telling the story of the original house designed by architect Richard Neutra, the demolition and the replica.

The commission directive, unprecedented in San Francisco, comes more than a year after the home at 49 Hopkins Ave., known as the Largent House, was almost entirely knocked down. All that remained of the white, two-story redwood-and-concrete-block home was a garage door and frame.

Johnston had received planning permission only to remodel with a design that would have largely kept the first floor of the existing home intact.

Two months after the demolition, Johnston applied for a retroactive demolition permit and for permission to construct a new home that would increase the size from about 1,300 square feet to nearly 4,000 square feet.

The case attracted attention because Neutra is considered one of the most important modern architects and because it highlighted the trend of speculators illegally razing modest homes with the intention of replacing them with mega-homes. The new houses can fetch upward of $5 million, double or triple the price of an average house in already expensive San Francisco.

Planning Commissioner Dennis Richards said he hopes the commission’s action in the 49 Hopkins case will send a message to speculators accustomed to ignoring city planning and building laws with few or no repercussions.

“We are tired of seeing this happening in the city and are drawing a line in the sand,” said Richards. “You can have all the rules in the world, but if you don’t enforce them, the rules are worthless.”

Justin Zucker, attorney for the property owner, said that 49 Hopkins LLC is not a real estate speculation group but an entity solely owned by Johnston, who had hoped to move his family into the larger home. Johnston’s LLC bought the home for $1.7 million in 2017.

Johnston briefly addressed the commission, saying that he had bought the property “as a family home that would enable my family of six to move back to San Francisco,” he said.

“I have been stuck in limbo for over a year,” he said.

Zucker argued that the historic integrity of the Neutra design had been erased over time — first in a 1968 fire and later in a series of remodels in the 1980s and 1990s. The approved 2014 plan — proposed by a previous owner — allowed for the removal of most of the existing structure, he said. “We acknowledge and apologize for the fact that a small portion of the work exceeded the scope in the approved plans,” he said, adding that the decision was made “for life-safety reasons.”

The decision comes a few days after Supervisor Aaron Peskin introduced legislation designed to crack down on illegal demolitions. That bill, the Housing Preservation and Expansion Reform Act, increases fines for illegal demolitions and requires a conditional use authorization for any home expansion that increases the square footage by more than 10 percent.

Peskin said that he was “very impressed” by the Planning Commission’s vote.

“The fact that it was a unanimous vote should send a message to everyone that is playing fast and loose that the game is over,” said Peskin. “We want to preserve iconic, historic structures, but even more important, we want to protect our reservoir of more affordable housing stock. You want a 1,300-square-foot house to be worth what a 1,300-square-foot house is worth, rather than a mega-mansion.”

While replicas are controversial among architectural historians, the Planning Commission decision was applauded by historic preservationists. In a statement read at the commission meeting, SF Heritage Executive Director Michael Buhler said that approving the proposed project would have “sent a strong message that existing planning and building laws can be ignored and there will be no repercussions.”

“The question before you once again is whether a person can demolish existing housing stock with impunity and then be rewarded,” said Buhler.

Planning Commissioner Kathrin Moore said she is confident that a replica could be “executed beautifully in a way that would be consistent with the home’s original expression.”

Neutra, who did most of his work in Southern California, designed five San Francisco homes. The Largent House was designed for a husband and wife who were teachers and artists. Neutra was known for his obsessive attention to the needs of his clients, whether it was a multimillion-dollar home or a modest structure like the one in Twin Peaks.

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