Office Hiring Seen as Slow in 1992 : * Employment: Only 20% of state firms plan white-collar additions, a survey shows. The figure was 21% a year ago.

One-fifth of California’s employers expect to add office workers in the first half of 1992, down slightly from 1991 and the lowest level in seven years, a survey suggests.

The survey conducted for the Irvine-based Thomas Temporaries employment agency also found that 13% of the state’s employers plan to cut their office staffs in early 1992, roughly in line with what the poll projected a year ago for early 1991.

Thomas’ survey, based on interviews with 1,845 California hiring executives, found more gloomy employers in Los Angeles County than anywhere else--18% of about 100 executives interviewed in the county said they would shrink their office staffs.

Widespread staff cuts are also expected in such areas as Sacramento and South Orange County.

The biggest hiring gains in the first half of the new year are expected in the East Bay/Oakland section of Northern California, where 29% of executives surveyed said they would hire people. Better-than-average increases in hiring also were projected for Riverside County, Contra Costa County and North San Diego County.

A year ago, 21% of employers were projecting adding office staff, versus 20% this year, the lowest level since Thomas began conducting its survey seven years ago.

Thomas Temporaries said it doesn’t know why the East Bay should have such an optimistic outlook or why Los Angeles employers are so downbeat.

“Just venturing a guess, I’d say there’s a greater dependence on depressed industries like banking and aerospace in Los Angeles,” said Gene C. Wilson, Thomas’ president. As for the East Bay, he said, “last year’s outlook was so negative that this year’s may be a reaction to that.”

The survey, which has a margin of error of 3.5 percentage points, found that general clerks and data entry clerks will be the office workers most in demand. About 12% of the executives said they would hire such workers.

Also in demand will be accounting clerks and secretaries with skills in information processing.

Government and medical businesses plan to do the most hiring this year, according to the survey. Wholesalers, legal and construction firms plan to do the least.