The D.C. Housing Department, faced with a chronic inability to collect rent from many public housing tenants, is exploring ways to take rent money directly out of the checks of tenants who are welfare recipients or employes of the city government, city officials told Congress yesterday.

Mayor Marion Barry told the House D.C. Appropriations subcommittee that the purpose of the plan would be to "get the money at the source, out of the welfare checks," before the recipients could spend it for something else.

Barry and City Administrator Elijah B. Rogers stressed that the rent-collection plan has not yet been adopted as policy. Deputy Housing Director James E. Clay, standing in a corridor of the Capitol, was talking to reporters about the details when Rogers broke in to remind him that Barry had emphasized the tentative nature of the proposal. "This hasn't crossed my desk yet," Rogers said.

The District has long been unable to collect all the rent due from the occupants of its 12,000 public housing units. This year, according to figures in the 1981 supplemental budget that the subcommittee was considering, receipts will be about $3 million less than the $13 million originally projected.

That figure is a drop in the bucket in an overall $1.5-billion operating budget, but the Barry administration is under congressional pressure to collect every cent that is due the cash-strapped District government.

Barry said it was "very hard" to collect rents through court proceedings because D.C. courts require landlords to demonstrate that their units meet housing code standards, which many of the city's units do not. Even if the courts would evict nonpayers, Barry said, he does not favor evicition as a general policy because "these people are already living in the most subsidized housing you have. Where are they going to go?"

Clay said that 2,338 of the 12,000 units are occupied by families receiving Aid to Families with Dependent Children, the largest welfare program. He said he had not yet determined how many units are occupied by District government employes, whose rent could be withheld from their paychecks.

Any attempt to withhold rent money from welfare checks might conflict with federal regulations, welfare law experts said yesterday. Federal rules limit the circumstances under which welfare funds may be taken out of a family's hands and paid directly to suppliers. They also permit the clients to claim that the supplier -- whether of medical care, housing or any other service -- has not honored his contract. In the case of public housing, where many units do not meet the requirments of the D.C. housing code, tenants whose rent was withheld from their checks would surely raise that claim in court, welfare experts and antipoverty lawyers said. 4tThe rent discussion arose when committee members inquired about Barry's proposal to add $3.3 million in general tax funds to this year's housing department budget, enabling the department to make up for the uncollected rents.

The subcommittee chairman, Rep. Julian Dixon (D-Calif), asked why the difficulties of collecting rent had not been factored into the original budget so that the revenue projections would be realistic. "You must have taken this into consideration," he said.

In a burst of frankness, Clay said that the Housing Department had used the unrealistic revenue estimates because that was the only way to balance its budget on paper. "We backed into it to balance the budget," he said. "The budget is balanced, but it bears no relation to the actual situation."