Virginia State Attorney General Anthony F. Troy has issued an advisory opinion that it is illegal for the state's localities to construct any public buildings under a procedure that Fairfax County school officials say has saved them both time and money.

Troy's opinion is a major victory for the state's independent general contractors and the latest skirmish in a national controversy over what builders call a "design-build" concept Fairfax County has used this concept in constructing three schools.

Traditionally, a school system hires an architect to design a school and then asks contractors for competitive bids on the cost of building it. Under the "design-build" concept, however, a school system announces what kind of a school it wants (such as a school for 100 handicapped students) and the maximum amount of money it has to spend. Then architects and builders form teams to propose the best school for the money and the school system makes its selection.

Fairfax school officials said yesterday that "design-build" projects not only can save money and time, but also can provide the school system with more imaginative and creative solutions to building problems. "This way you can get 12 solutions to your problems," said John Krytusa, management operations director for Fairfax schools. "With the conventional approach, you get only one solution to your problem."

But Troy, citing a Virginia law requiring competitive bidding on public projects costing more than $2,500, has said that the state's localities must follow the conventional practice.

Under Virginia law localities are prohibited from doing anything not specifically allowed by the state legislature.

Although that it is the procedure Fairfax has used for most of its school construction, it frequently can cause delays when bids come in over the targeted price of the school, Krytusa said. As a result, Fairfax school officials and officials in many other jurisdictions have turned to "design-build" structures, with a set maximum cost on a project.

That procedure has angered contractors, who argue that they cannot compete fairly on the various designs, and architects, who contend the process places them in a conflict of interest. Under the rules of the American Institute of Architects, its 26,000 member architects are not supposed to have any interests in the construction firms that build their projects.

Spokesmen for both the AIA and the Associated general Contractors of America, Inc., a trade group, said yesterday that despite their groups' opposition to the "design-build' concept, it is gaining rapid acceptance around the country. The AIA in June rejected a move to accept the practice and the contractor's group, which supports the practice in private industry, is currently reviewing its opposition to public agencies using the "design-build" concept, the spokesmen said.

Troy's four-page opinion caught Fairfax school officials by surprise. They said yesterday they would have to study it before they could comment.

To date the county has been "well pleased with the results" of its "design-build" schools, Krytusa said.

Even though the Troy opinion is only advisory, William H. King Jr., a Richmond lawyer who submitted a legal brief for the contractors to Troy's office, said yesterday such opinions "are usually given great weight by the courts" and are seldom overturned. "What it does do is tell the state's municipalities that this method of awarding bids for public buildings is improper." King sad. King said he could not rule out the possibility of a law suit if other localities attempt to use the "design-build" concept.

One reason Fairfax officials were unaware of the opinion, issued Sept. 8, was that it came in response to a request from State Sen. Stanley C. Walker, a Norfolk Democrat and chairman of a structural steel company. Officials of a Virginia contractors group said they had asked Walker to seek the opinion.

The contractors' group has no plans to contest the recent award by Fairfax of a $1.6 million "design-build" contract for construction of a special educationcenter at the Kilmer Intermediate School near Tysons Corner, King said. Krytusa said yesterday that is the only current project the county school system has under construction under such a contract and that no additional projects are now planned under such contracts.

In his opinion, Troy said. "Under the 'fixed price design-build' procedure there is no low bid (as required by state law). Each bidder, offers to build his own design for the price previously fixed by the owner."