Fairfax County economic officials have reported record-breaking commercial and industrial construction figures for fiscal 1984 and are predicting another banner year for fiscal 1985.

Commercial and industrial construction hit a record high of $399 million during the last fiscal year, according to the annual report released by the Fairfax County Economic Development Authority (EDA).

"All the trends so far for the first four months of this fiscal year predict this could be as strong a year as the last," said April Young, executive director of the EDA. She said a continuing decline in interest rates and current federal policies in the Washington marketplace are major reasons for the prediction of "continued growth in this marketplace." Those federal policies include an increase in defense contracts given to area firms.

Fairfax County's fiscal year runs from July 1 to June 30. The report was released in late October.

The $399 million figure represents a 31 percent increase over the previous record set in 1983, according to Charles G. Gulledge, chairman of the EDA.

Commercial and industrial growth in fiscal 1984 generated more than $14.6 million in new taxes for Fairfax and added more than 14,000 jobs to the county's employment base, the report said. Both of those figures set records for Fairfax.

The growth figures have pleased members of the Fairfax County Board of Supervisors. Board Chairman Jack Herrity said the figures mean the county is moving steadily toward a goal set in 1977 of increasing the proportion of its real estate revenues generated by industrial and commercial development from 12.7 percent to 25 percent. For fiscal 1984, the commercial/industrial base accounted for 18.5 percent of the tax base.

EDA officials pointed to growth in the first half of 1984, the last months included in the fiscal year report, as encouraging for the 1985 building period. The value of nonresidential construction more than doubled from the value of what was built during the first half of 1983.

The report's statistics do not reflect major projects that recently have been approved or are in the planning process, including the 107-acre Tysons II mixed-use development near Tysons Corner or TRW Inc.'s Defense Systems Group's 120-acre complex planned at Fair Lakes in the Fairfax Center area.

However, projects under construction in fiscal 1984 totaled 6 million square feet, the largest ever in Fairfax County. EDA officials estimated that 2.3 million square feet of office space was finished during the year, "with an unprecedented 2.8 million square feet being absorbed," according to the report.

EDA officials reported a vacancy rate of 5 to 7 percent and a total office inventory of more than 32 million square feet. Although that vacancy rate would delight some officials in some of the nation's major cities, several Fairfax officials said they hope the figure will drop even more despite the fact that additional office space is planned throughout the county. Generally, EDA officials have predicted that Fairfax will absorb enormous amounts of office space that major mixed-use developments are expected to generate in the next few years because all of the space is not expected to hit the marketplace at the same time.

"Major leases signed last year included Hazeltine Corp., AT&T, Advance Resource Technology, ATEX, Xerox, GTE Sprint, Shared Medical Systems, Booz Allen, MA/COM Linkabit and MCR," the report said.

The EDA report said that more than 1.25 million square feet of what it calls "industrial/hybrid space" was constructed, boosting the total of existing space in that category to 20 million square feet. EDA defines "industrial/hybrid space" as buildings used for research and development, high technology, light manufacturing or assembly, warehouses or combinations of these uses.

The growth of commercial and industrial space in the county is generating an increase in hotel and motel space. More than 1,400 new rooms are under construction, and another 1,400 are planned for completion by fiscal 1986. This figure does not include hotels or motels recently approved or several major hotel projects that are in preliminary planning stages.

Fairfax County spent $1.3 million to attract business during the last fiscal year. Those dollars produced a return of $10.48 for each dollar spent, the report said.

Although the $30 million Governor's Center for Innovative Technology has been a factor in many recent decisions by major corporations to locate in either Fairfax or Loudoun counties, Young predicted that Fairfax will not feel the full impact of industrial and commercial development that project will generate until at least 1986.

According to the EDA report, almost 35 percent of the companies now seeking space in Fairfax are from outside the Washington metropolitan area. This is almost double the non-local growth experienced by communities nationwide, the report said.

Almost a third of those non-local companies are now based outside the United States, the report said.

The most expensive building listed as being built during fiscal 1984 is a $39 million, 470-square-foot Continental Telephone facility near Dulles Airport, which generated 1,350 jobs.

The largest project in terms of square footage was a 600,000-square-foot, $25 million facility built by Computer Sciences Corp. at Merrifield. That project generated 800 jobs.