Major construction unions in the Washington metropolitan area -- under heavy competitive pressures from non-union workers -- have reached tentative agreement on a new contract within the Carter adminisration's anti-inflation guidelines, union officials reported yesterday.

John Quackenbush, secretary-treasurer of the Washington Area Building Trades Council, said the contract now must be ratified by the approximately 20,000 workers covered by the agreement. Quackenbush said the agreement involves "probably more crafts than labor agreements in any other industry."

There are 17 basic construction unions, covering workers in Maryland counties. The only area union definitely not part of the agreement is the International Union of Operating Engineers.

Federal mediator Charles Scott said "no other cases have passed through this office which involve such diverse groups."

Details of the agreement were not available immediately, but Quackenbush indicated the wage increase would be within the President's 7 percent guideline.

Last year, the unions worked out an unprecedented contract in which the various unions agreed to work rules changes and signed a no-strike pledge in an effort to cut costs and increase productivity. They also agreed to extend existing wage and benefit rates for one year, ending April 30.

Quackenbush indicated the unions might have trouble winning membership ratification of the new agreement because of the limits on wage increases. "They went a whole year with no raises and it didn't even seem to make a drop in the bucket" in holding down inflation, he said.