From the beginning, there has been a hidden conflict in the administration's welfare reform planning. Its proposal, as laid out for President Carter recently would expand the welfare system to include new groups particularly more of the working poor. On the other hand, however, is Carter's edict of not adding federal dollars to the current $25 billion welfare system . . . at least not at the outset.

To expand the system without extra federal money means some welfare recipients must get less than they do now or some states must pay more.

Welfare experts - within and outside the government - remain skeptical, critical and uneasy about what they term a financial "juggling act" and about many of the money-saving options Carter is examining before deciding on and presenting his proposal to Congress this week.

One crucial issue Carter has to decide is what constitutes a "family" eligible for welfare. The basic administration proposal would count all income earned by blood relatives living in a household when calculating welfare benefits.

It is estimated that this definition would save $1 billion over the present AFDC (aid to families with dependent children) program - which counts AFDC mothers and their children separately from other house hold members. But critics point out that it would also drastically cut back assistance to many of the poor.

A teenage mother with a baby and no income would become ineligible, for example, if she lived with an aunt whose earnings exceeded the welfare cutoff - even though the aunt is not obligated to support the teenager. Under the HEW suggestions, some sort of review procedure might be added so that such persons could file as two separate units in the same household if the aunt could prove she did not support her niece.

The administration plan also calls for extending the "accounting period" used to calculate eligibility for benefits. The current system operates on a "prospective" accounting period - with those seeking AFDC and food stamp assistance estimating an "anticipated" income for the next few months. If they are laid off, for example, and anticipate little or no income, they can get aids.

The administration contemplates changing this to a "retrospective" program that would compute what income a person actually had during the last six months.

This would reduce the numbers of eligible recipients and therefore save money. Critics charge that the destitute - a man with three children who lost his job and had no other income, for example - could have to wait between two and three months before receiving any assistance while he "spent down" any money left from his previous income.

A third set of issues before Carter has to do with jobs. The administration is promising 1.4 million public jobs for low-income workers at a minimum wage with a small percentage of supervisory jobs in excess of minimum. Union leaders and welfare rights advocates say the jobs should be at the "prevailing" wage.

There are more than 600,000 jobs now under the economic stimulus package and an additional 125,000 will be in place by October. These are temporary, usually for a 12-month period, and the average pay is $7,500 a year. The plan is that, as these jobs are phased out, the money would not disappear but would be used to help provide the 1.4 million minimum wage jobs.

Arnold Packer, assistant secretary of labor, admits that it is "likely that some who signed on for these CETA (Comprehensive Employment and Training Act) jobs, who would be phased out in a year, could end up taking one of the new (welfare reform) jobs at a lower wage."

Jim Savarese, director of public policy analysis at the American Federation of State, County and Municipal Employees, expresses the concern that "most of these million low-paid people would be dumped into the state and local sector. Image what that would do to wage conditions for city jobs. An analogy I can make is to put 100,000 of them in the auto plants a minimum wage and see how the auto workers would like it." Saverese contends they should provide fewer jobs that pay more.

The Labor Department contends the jobs would not conflict or compete with existing jobs, but state, county and municipal employees foresee an unavoidable overlap.

The Labor Department lists 1.4 million possible welfare reformed slots in 12 various job categories - from public safety to clean-up and pest control. It estimates nearly 140,000 slots in state, county and municipal parks, such as cleaning up playgrounds and baseball diamonds.

Other suggested jobs include installing locks and escorting senior citizens in high-crime rates, building ramps for the handicapped, air pollution monitoring, child care, expanded trash collection and installing storm doors.