President Carter said yesterday that one piece of his tax reform package to be submitted to Congress this fall will be the substitution of a $240-per-person credit for the existing $750 exemption.

He revealed the plan during a question-and-answer period with employees of the Department of Housing and Urban Development. The visit was one of a series Carter is making to major federal agencies to ask employees to make the federal government more responsive to the people.

The flat dollar amount (which would also replace an existing $35 credit per person) suggested by Carter yesterday would favor poor families at the expense of the rich. But Treasury sources noted that oter possible changes in the package could offset this particular shift.

Under the present system, a $750 personal exemption is worth $375 in reduced taxes to somcone at the upper income level who pays taxes at a rate of 50 per cent. But it is worth only $105 to someone in the lowest, 14 per cent bracket.

Thus, a family of four in the 50 per cent bracket saves $1,500 in taxes through the exemption procudure. A family of four in the 14 per cent bracket saves only $420.

Under the credit system, Treasury officials said, taxpayers below an estimated $20,000 to $25,000 adjusted gross income would benefit, while those above that range would pay higher taxes (unless other changes are made).

However, the credit is not refundable: it could be claimed only up to the level of a taxpayer's tax liabilities.

The Senate Finance Committee has recently consideredd and rejected the personal credit, including a proposal along the Carter lines by Vice President Mondale, who was a member of that committee as a senior.

Carter, in answering the question, yesterday, misheard the questioner, who asked about the mortgage interest deduction. In his reply he talked about the standard deduction, but aides later said that it was the personal exemption he had intended to discuss.

The President said that the move toward a personal tax credit, rather than exemption, was originally considered for inclusion in the economic stimulus program, "but we couldn't afford it."

Tax experts cautioned that it was much too early to get a general sense of what the whole Carter reform package might look like from this one piece. But they suggested it was significant that the President and his advisers appear to be thinking in terms of credits, which favor low-income families, rather than exemptions, which are considered "regressive," favoring high-income families.

Carter was greeted with enthusiastic applause when he entered the HUD cafeteria, filled to capacity by hundreds of employees, many of them there to see both the President and the new HUD Secretary, Patricia Roberts Harris, for the first time in person.

Carter reassured the bureaucrats that none of them would lose his or her job - or lose seniority - as a result of federal government reorginization, one of the President's campaign promises and a favorite theme since he entered the White House.

On a different topic, the President urged the assembled worker's "to protect the integrity of your families." And then he brought a roar of laughter and applause when he said:

"Those of you who are living in sin - I hope you'll get married. Those of you who have left your spouses - go back home. Those of you who don't remember your children's names, get acquainted. I'm very serious about that," the President said, smiling broadly.