In the latest sign of the astronomical cost of living in parts of California, the federal government now classifies a family of four earning up to $117,400 as low-income in three counties around the Bay Area.

That threshold, the highest of its kind in the nation, applies to San Francisco, San Mateo and Marin counties. It’s used to determine eligibility for federal and local housing assistance programs. (But it’s different from the federal poverty guidelines.)

To generate the number, officials at the Department of Housing and Urban Development factor in the median income and average housing costs in an area. The second-highest threshold is in Honolulu, according to the agency — but the third is also in the Bay Area, in Santa Clara County, the heart of Silicon Valley. The New York City area, where a family of four earning up to $83,450 is classified as low-income, came in at No. 9.

Back in the Bay Area, residents and experts said they weren’t surprised.

“It’s arguably the most expensive city in the country, so what that translates to is really not that much money,” said Ed Cabrera, a HUD spokesman who is based in San Francisco. “Especially with children in an area where properties are considered affordable if they’re going for half a million dollars.”

The federal government pegs the fair market rent for a two-bedroom in the San Francisco area at $3,121. The median home price has climbed above $1 million.

The “low income” designation allows people to qualify for affordable housing and a variety of government programs, such as those for first-time homebuyers. But officials noted that a vast majority of San Francisco-area residents who get direct housing assistance, such as vouchers known as Section 8, are well below the maximum low-income standard: The average household that receives assistance makes just $18,000.

In neighboring San Mateo County, officials say the housing stock lags far behind demand. Many residents who have been forced to move farther inland now face grueling commutes to their jobs.

Kate Hartley, director of the San Francisco Mayor’s Office of Housing and Community Development, said high construction costs and low federal funding had added to the challenges of keeping low- and middle-income people in the city.

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