The Fairfax County Board of Supervisors moved a step toward setting up a county-operated bus system south of Alexandria yesterday when it decided to seek a consultant for the project.

The board, which has delayed action for weeks on the bus project, deferred the controversial question of whether the bus routes should follow existing Metro routes or should go onto less-traveled neighborhood streets.

Under the proposal, Fairfax will hire a transportation consulting group to develop cost estimates for serving the Huntington Metro Station, which is scheduled to open in December. The county also could ask the consultant to operate the bus system day to day.

Since the supervisors voted in October to set up the county bus system, the project has languished. Supervisor Nancy K. Falck of the Dranesville District and other supervisors have complained that if the project is not closely controlled, it could pour money into one area while doing nothing for other parts of Fairfax. The county staff estimates that the buses will begin operating about 1985 at a start-up cost of $6 million.

Supervisors at yesterday's meeting questioned whether the staff proposal to advertise for a consultant adequately reflected the board's original plans for the system. "That wasn't what I thought we said," Falck said after the presentation by Shiva K. Pant, county transportation director.

Despite some misgivings, the board voted by voice vote to allow the staff to proceed.