Engineers have proposed an unconventional solution to the Millennium Tower’s tilting troubles: drilling piles down to bedrock to stabilize one side of the 58-story condo high-rise and then letting the other side continue to sink until the building straightens itself.

“The engineers are very confident this is going to stabilize the building,” said Vision Winter, a partner at O’Melveny & Myers, the law firm hired by the homeowners association to obtain funding for the building’s mega-repair.

But it won’t come cheap.

Experts estimate the job could cost from $200 million to $500 million — in other words, probably as much as the $350 million it cost to build the tower in the first place.

Soil tests for the proposed retrofit already are under way.

The Millennium Tower, which opened with great fanfare in 2009, has sunk about 17 inches and tilted 14 inches to the west (toward Fremont Street) and 6 inches to the north (toward Mission Street) at the roof line. And it’s continuing to sink and tilt.

Ever since the scope of the problem was disclosed two years ago, the blame game has been in high gear — with fights featuring the developer, the designers, the city and the Millennium’s neighbors, who dug up blocks of downtown for their own projects.

The building sits on a 10-foot-thick mat foundation, held in place by 950 reinforced concrete piles sunk 60 to 90 feet deep into clay and mud. They do not, however, reach bedrock.

To retrofit the tower, engineers are proposing boring 275 to 300 steel and concrete “micro piles” — each 13 5/8 inches in diameter — down to bedrock. But only on one side of the building at a time.

The first step would be for crews, working inside the Millennium basement, to drill about half the new piles on the building’s west side.

Once it’s stabilized, workers would allow the east side to sink long enough for the building to straighten itself. Only then would the rest of the piles be drilled to bedrock on the east side to keep everything from sinking even farther.

The entire process could take two to five years.

According to Winter, the building’s general contractor and engineers are already working with the city to obtain needed approvals and permits for the work, and he’s hopeful they’ve found a solution.

But others aren’t so sure.

“This is not a fix,” said attorney Frank Pitre, who represents 85 Millennium condo owners apart from the homeowners association.

“At best, this is going to arrest further tilting of the building,” Pitre said.

David Casselman, the Los Angeles lawyer representing 265 owners separately from Pitre, said the only real fix for the tilting tower would be to tear it down and rebuild it.

“But that probably would cost a couple billion dollars,” Casselman said.

Casselman is suing the Transbay Joint Powers Authority, the public entity that is wrapping up construction on the $2.2 billion Transbay Transit Center, which is next to the Millennium.

He says he has proof that the authority — composed of San Francisco, AC Transit, the Peninsula Corridor Joint Powers Board, the California High Speed Rail Authority and Caltrans — ignored its legal obligation to prevent the residential high-rise from sinking when it began excavation work on the huge bus and rail hub in early 2012. And he expects the authority to pay.

Casselman and Pitre also both worry about the disruption and inconvenience to homeowners while the proposed repairs take place.

“They’re telling people that there won’t be any issues of noise or vibration, and I found that astounding given the magnitude of the work they’re taking on,” Pitre said. “Construction management has told us they could have 300 trucks — dump trucks and cement trucks — over time going through here.”

Plus “nobody is issuing any warranties on the work,” Pitre said.

Plaintiffs also worry about the proposed repair making a bad situation worse, particularly given the stress the building has already undergone — having shifted in two directions, allegedly as a result of the digging and “de-watering” soil for the neighboring transit center and Salesforce Tower.

“I’m no engineer, but this building is not made of rubber — it’s made of concrete,” said Pamela Buttery, whose two-bedroom unit on the 57th floor has sloping floors. “And no one has proved to me that the concrete pad is stable enough to withstand 300 micro piles going into it.”

Whatever the case, in a matter of days, drilling equipment is expected to be rolled into place outside the Millennium to bore a pair of holes deep into the ground to test the conditions of the soil and bedrock below. As part of the job, crews have already begun tearing out a portion of concrete sidewalk and trees along Beale Street.

Before the full repair job gets going, however, lawyers and insurance representatives for all the feuding parties — developer Millennium Partners, the Transbay Joint Powers Authority, the Salesforce Tower’s owners, the city, plus all their engineering and design consultants — will try to hammer out a comprehensive agreement to pay for anchoring the building and settling the multitude of condo owner claims.

But P.J. Johnson, a spokesman for Millennium Partners, which is funding the soil testing, called it “an important step forward in the effort to protect the building from further harm.”