As empty buildings continue to mount in the region, office vacancies in the Bay Area’s biggest markets have reached a grim milestone — hitting a record high, new reports show.

Silicon Valley, which the report’s authors consider roughly equivalent to Santa Clara County, downtown Oaklandand San Francisco all set records for office vacancy rates in the most recent three-month period that ended in September, commercial real estate firm JLL reported in separate surveys of those markets.

Tenants continue to seek ways to reduce their corporate footprints, a dynamic that is keeping office vacancies at brutal levels.

Here are the details for each market in the third quarter:

• San Francisco, which is locked in what numerous experts believe is an economic “doom loop,” posted a third-quarter vacancy rate of 34.5%.

• Downtown Oakland’s office vacancy rate was 29.1%.

• Silicon Valley reported an office vacancy level of 22%.

Despite the ominous statistics, Alexander Quinn, JLL senior director of research for Silicon Valley, says some signs of hope have begun to emerge for the battered Bay Area office markets.

“Some things are happening in Silicon Valley that give us an expectation for a recovery,” Quinn said. “Tech companies have greater conviction they can get their employees back into the office. Employers believe they can be more forceful about getting employees back into the office.”

With the job marketing marketing softening, workers may be more inclined to heed prompts from supervisors about being in the office for more hours. “Employees may be more willing to endure their commutes,” Quinn said.

There have also been encouraging gains in tenant leasing, especially in the San Jose Mineta International Airport market and in the city of Santa Clara, JLL reported.

“Leasing activity in Silicon Valley is up 21.6% from the previous quarter,” JLL reported in its assessment of the South Bay office market for the third quarter.

But in downtown Oakland, the July-through-September quarter was bleak, with little room for optimism. Downtown Oakland’s office market was sluggish at best.

Leasing activity, the number of rental deals and the average lease size declined in downtown Oakland during that period compared with the previous quarter.

The city’s downtown faces a significant challenge due to huge blocks of vacant office space.

“Two more full floors came to the market this quarter” in downtown Oakland, JLL reported. “Clorox listed another floor for sublease at 1221 Broadway and APEN’s former space at 426 17th Street was listed. This brings the total number of full floors available to 133 in downtown Oakland.”

If a typical Oakland office highrise is 20 stories high, 133 empty floors could equate to six or seven completely vacant office towers in downtown Oakland.

“We are seeing statistics that show an improvement in the safety of downtown Oakland,” said Quinn, citing figures from the Oakland Police Department.

San Francisco is — by far — the worst of the three office markets, with a vacancy rate that is 5 to 12 percentage points higher than downtown Oakland or Silicon Valley.

“Vacancy increased to 34.5%” in San Francisco, “largely due to continued consolidation” by office tenants in the city’s Financial District, JLL reported.

Even worse, office rental rates are particularly weak in San Francisco. Rents are roughly 33% below the levels seen in 2019, the final full year before coronavirus-spawned business shutdowns in 2020.

Still, it appears that workers are starting to spend more time in San Francisco, according to cell phone activity figures that JLL experts have reviewed.

“We are seeing reports of a 13% increase in the amount of activity in downtown San Francisco compared to a year ago,” Quinn said.

The JLL report offered some hope for these three key office markets — although the reports warned that any real improvement in vacancy levels won’t materialize before 2025.

“Return-to-office rates have trended upward, 6% higher than this time last year” in San Francisco, JLL reported. “Remote job postings are also down 16% year-over-year. Both indicate that companies are shifting away from a remote-friendly work environment.”

Some encouraging signs for downtown Oakland have emerged due to government entities seeking to rent or own office spaces in the East Bay city’s urban core.

“Downtown Oakland has seen stabilization among its public sector tenants, including major commitments from BART PD, the FBI, and FEMA,” JLL reported. “As remote work mandates shift, so will workweek activity shift in downtown Oakland.”

Silicon Valley is starting to see a big increase in tenant demand as companies scout for office space to a greater extent, JLL reported.

“JLL is tracking approximately three million square feet of office requirements, a 21.4% increase” in the third quarter compared with the second quarter, JLL reported, with more tenants scouting for much larger spaces in the July-through-September third quarter than they did in the April-through-June second quarter.

“While smaller requirements see higher demand and activity, 100,000-plus square feet requirements have tripled this year, signaling potential new deals,” JLL stated.

Not all office markets will bounce back at the same speed, said Bob Staedler, principal executive with Silicon Valley Synergy, a land-use consultancy.

“Silicon Valley vacancy levels will lower faster than San Francisco and Oakland,” Staedler said.