Not long ago, office space in Fairfax City consisted of less than a dozen one-and two-story houses built in the 1800s and the early 1900s and leased mostly to lawyers practicing in the nearby county courthouse.

Today, the once somnolent town of Fairfax finds itself in an office boom - despite the avowed intention of the Fairfax County Board of Supervisors to move the city's major industry into the county.

There's the five-story L.F. Jennings Building under construction on Chain Bridge Road cross from the site of the new courthouse, which isn't under construction yet.

Two doors down, workmen are putting the finishing touches on additions to the off-white Barbour House. In front of the building is a sign - "Office Space for Rent."

Across from the old courthouse is the newly built, five-story First Federal Plaza owned by First Federal Savings and Loan Associates of Arlington, which moved its administrative and executive offices from Arlington to the city in February.

Two blocks away, Courthouse Plaza, a complex of two-story office buildings and town houses, is being built.

In all, 12 office buildings, totalling 620,000 square feet, are under construction or are awaiting approval of site plans for construction.

"We're exploding," said Harry Cologne, the zoning administrator for Fairfax City.

Cologne said the city has doubled its office space - the city had 300,000 square feet prior to the office boom - within the past year.

Fairfax City, a six-square-mile community with a population of 22,000 located in the center of Fairfax County, has become an attractive area for developers hoping to cash in on the increased growth and activity in the outlying suburbs.

"We feel like the city is going to grow," said Larry Jennings, vice-president of L.F. Jennings Inc., which is construction the L.F. Jennings Building.

Since 1972, the city has been its employment force increase in number. According to the latest figures, there are 17,667 jobs within the city, an increase of 1,383 since March, 1972.

The largest category of employment in the city is the Fairfax County government with its 6,729 employes.

Most of the county employes are housed in the 12-story, box-like Massey Building, which towers over all the buildings in the city. Office buildings in the city are limited to six stories, except for the Massey Building.

However, the possibility that county officials may move the county offices out of the city does not appear to be a major concern of developers and new businesses in Fairfax City.

"The city itself and the courthouse will generate enough activity," said Jennings.

"We did not move here because of the county offices," said Allan R. Plumley Jr., president of First Federal Savings and Loan of Arlington, which still maintains its main office in Arlington. He said First Federal moved to Fairfax City because the company needed more space.

Most developers say they are having no problem finding tenants for their office buildings.

Alan Kay, president of Rozansky and Kay, a Bethesda firm that is building Courthouse Plaza, said he already has 80 percent of his tenants and expects to fill the other 20 percent within 60 days.