President Carter's proposal to set a national income of $4.200 a year for a typical family of four was tentatively adopted with hardly a word of debate yesterday by the special House subcommittee dealing with his new welfare plan.

The subcommittee also voted to make welfare benefits subject for the first time to federal, state, and local taxes, and to ease eligibility requirements by changing the way income was to be counted in the Carter plan.

The eligibility changes junked one of the major devices the administration was using to keep costs down - measuring an applicant's income over the preceeding six months instead of counting just the last month, as is now generally done.

Dan Marcus, deputy general counsel for the Department of Health, Education and Welfare, said the eligibility changes would make 8.8 million more people eligible than the Carter plan and would have added $1.7 billion to the estimated cost of the program.

Some of that, he added, would be returned through the income tax on welfare benefits.

The subcommittee approved by voice vote almost all of the Carter plan's dollar benefit levels, most of which had been critized repeatedly in public hearings as being top low or too high.

Those levels include $1,100 a year for a single individual; $2,200 for a childless couple; $2,500 for an aged, blind or disabled individual, and $3,750 for aged, blind or disabled couple.

The $4,200 for a family of four with dependent children is more than such families now get in 10 Southern states and more than the federal share of current welfare and food stamp benefits to such families in all but seven states.

The administration has said it expects most states now paying benefits higher than $4,200 to supplement payment, and that the federal government will pay a share of these supplements as an incentive to the states.

Benefits to the aged, blind and disabled remain about the same as in the current remain about the same as in the current Supplemental Security Income (SSI) plan, while single individuals and childless couples would be covered for the first time in a federal cash program.

The only federal program they are now eligible for is food stamps.

The subcommittee voted decisively twice last week not to raise benefit levels approximately 33 per cent more than Carter proposed, up to the poverty line.

There was some speculation that yesterday's vote, called unexpectedly as a tired subcommittee neared the end of its workday, was a hedge to prevent conservatives from trying later to lower benefits to bring down the program's overall cost.

"I tell you, we have our work cut out for us when we promise we'll improve the lot of everyone on welfare, give fiscal relief to cities and not spend any more money," said Chairman James C. Corman (D-Calif.). "The President came within $2.9 billion," of his original cost ceiling. "We should come within $2.9 billion of that."

In a series of other votes, some of them roll calls, the subcommittee:

Refused to adopt a so-called "paperboy amendment" that would let students hold part-time jobs without having their earnings reduce their parents' welfare payments. Corman said the issue would be reconsidered today if more safeguards could be found to head off potential abuse.

Retained a provision in current law that aged, blind and disabled people be checked for eligibility every three months, instead of every month.

Changed the assets test proposed by Carter to the one now used for aged, blind and disabled individuals.

Under this, a single person would be barred from benefits if his assets exceeded $1,500. The figure would be $2,250 for a couple. Homes, household goods and a car would not be counted. Carter wanted to raise the cutoff figure to $5,000 for an individual, but to count 15 per cent of all assets between $500 and $5.000 as income. A subcommittee staff member said states complained that would be "an administrative nightmare."

Refused to raise the assets cutoff, which was set as its current lvel in 1971, by the rise in the cost of living.

The proposal to tax welfare benefits was made by Rep. Barber B. Conable Jr. (R-N.Y.).

"It just seems to me you're not asking too much of people on welfare that they pay taxes it because working people would," said Corman.

Rep. Fortney H. Staek (C-Calif.) proments. Administration officials argued that the 8.3 million more people who would be made eligible by Stark's proposal "would be people whose incomes are high on an annual basis, but which fluctuate throughout the year . . . construction workers, seasonal workers."

That was disputed by other welfare experts.

Figures compiled by the Cneter on Social Welfare Policy and Law indicated that a family of five earning $10.800 a year would have to wait three months after becoming jobless before getting any benefits under Carter's eligibility rules.

Those benefits would be only $80 the first month, and $160 the second, the figures showed.

HEW officials said they had no estimate of how much yesterday's votes would add to the cost of the welfare plan. A subcommittee staff member said, however, that members may reconsider some of their decisions if the cost seems too high after all the votes.