Congress must make two major retirement decisions soon that will have an immediate impact on 700,000 civil servants, ranging from longtime employes to those who have just begun their careers.

The most pressing issue is creation of a new retirement plan for the 300,000 people hired since 1984 who are covered by the civil service pension plan and Social Security.

That interim agreement is due to end next month. Unless a new system is in place or the current system extended, those employes would have twice as much money, or more than 14 percent, taken from their checks starting in June to pay for full Social Security and civil service coverage.

They currently pay the full Social Security tax of 7 percent but only a token amount, less than 2 percent, for civil service coverage. The latter coverage for workers hired before 1984 costs them 7 percent.

The Senate and House have approved new retirement plans basing benefits on Social Security, civil service and a voluntary tax-deferred investment program.

The House bill would cost 25 percent of payroll, the same as the plan covering pre-1984 hires. The Senate plan, with less generous early retirement options and reduced cost-of-living adjustment provisions, would cost 21.9 percent. The cost difference is several billion dollars a year.

Administration sources have said that anything more costly than the Senate plan faces a veto.

Staff members from the Senate Governmental Affairs Committee and the House Post Office-Civil Service Committee have worked out a tentative retirement plan. But its cost exceeds the minimum figure laid down by the White House.

The compromise plan will soon go to members of the two committees, who are expected to approve it in the face of deadline pressure.

The Senate and House probably will go along with whatever the conferees approve. The next step would then be up to the White House.

If the president approves it, the new system would go into effect this summer. But if it is vetoed, new employes would have a much bigger bite taken out of their checks to pay for Social Security and civil service retirement coverage.

The other retirement issue facing Congress is an early-retirement bill introduced by Governmental Affairs Committee Chairman William V. Roth Jr. (R-Del.) and ranking majority member Ted Stevens (R-Alaska).

The bill would open an early-retirement window between July 1 and Dec. 31 for thousands of workers not eligible to retire under current rules.

The plan under consideration, designed to cut the federal work force without budget-triggered layoffs, would let workers retire at any age after 25 years of service; at 55 after 15 of years or at 57 after only five years of service. Benefits would be reduced 2 percent for each year the retiree was under age 55.

Under current rules (which set a minimum retirement age of 55 after 30 years) about 200,000 people are eligible to retire. Roth-Stevens bill would make an additional 200,000 eligible during the six-month period.

Many federal workers welcome the Roth-Stevens plan. But it is unlikely that it will be passed by July. If the White House agrees to it, or some kind of compromise, chances are the early retirement window won't be opened until later this year or even January.