Sidney R. Steele, Fairfax County's chief planner, resigned yesterday to take a higher paying job in private industry, continuing what some officials said is a troubling trend among senior county employes who are familiar with the county's building boom.

Steele, a 12-year county employe who has headed the Office of Comprehensive Planning since October 1984, delivered the unexpected news to officials in a letter that stressed that his departure was not prompted by friction within his department or by dissatisfaction with his $57,285-a-year job.

Contacted at a planning conference in Boston, the 56-year-old Steele said he resigned for "a variety of personal, family and professional reasons." He cited his salary as a primary reason for the decision, saying he has been promised "a substantial increase" from his new employer.

Steele, whose resignation is effective Jan. 3, declined to identify the firm other than to say it is located in Fairfax.

Steele thus joins a growing list of county planners, transportation specialists and public works employes who have left the county in recent months to assume more lucrative positions in the private sector. County officials estimate that during the last year alone, the planning department has lost about 10 specialists to private industry.

"It's no secret that we are being raided by other governments and by the private sector," County Executive J. Hamilton Lambert said yesterday. "They're paying better bucks."

Lambert characterized as "hot commodities" those county employes involved in matters concerning Fairfax's development boom, and he urged the Board of Supervisors to upgrade the county's pay and benefits package in an attempt to keep them. Lambert said private companies are luring away Fairfax County employes with pay offers that range from 20 to 40 percent above their government salaries.

"The board is going to have to take a strong look at the compensation package," he said.

Supervisor James M. Scott (D-Providence), chairman of the county board's personnel committee, said, however, it is unlikely that the board would support major changes in the county's salary and benefits package. Scott said he has asked the committee to review the employe compensation plan but that "there hasn't been a great desire to do it."

Although describing Steele's departure "as part of a trend" that has seen the county lose some of its best employes, Scott said he doubts the board will tamper with its policy of paying salaries that are in the middle range of what most area governments pay.