While hardly anyone was looking, Rep. Jamie L. Whitten (D-Mill.), powerful chairman of the House Appropriations Committee, recently slipped a giant goody for senators and representatives into an obscure appropriations bill.

The goody is a tax break for members of Congress, who have spent most of their time recently cutting federal budget provisions for things like food stamps and school lunches.

Members would be allowed to deduct living expenses here from their federal income taxes, just as businessmen now do while on the road.

The idea, Whitten says, is to "put members on the same footing as private businessmen" in deducting business expenses away from home.

The Joint Committee on Taxation estimates this could provide the typical member of Congress a deduction of about $13,500 a year and cost the Treasury more than $3 million in taxes.

Members, whose annual salary is $60,662.50, now are allowed to deduct $3,000 for living expenses, the figure set in 1952.

The exact provisions of Whitten's measure are unclear. Its language has not been publicized, and almost everyone familiar with the proposal is treating it in a very hush-hush manner.

Whitten, who was not available for comment yesterday, introduced the matter June 18 at the end of a markup session in the House Appropriations subcommittee on the legislative branch. No formal vote was taken, but subcommittee chairman Vic Fazio (D-Calif.) and other present quickly agreed to include the proposal in a $1.1 billion legislative appropriation bill.

The amendment is scheduled to come before the full committee when Congress returns from the July 4 recess next week. It is supported by the committee's ranking Republican, Silvio O. Conte (Mass.), and Dan Rostenkowski (D-Ill.), chairman of the House Ways and Means Committee, which normally has jurisdiction over tax matters.

Like any issue touching the members' pocketbooks, the Whitten proposal is hypersensitive on Capitol Hill. A few brave souls have tried for years to lift the $3,000 deduction limit, but penny pinchers have beaten them back each time.

Rostenkowski, who lives with his family in Chicago, proposed in 1979 that members be given a $50-a-day deduction while on public business. Few argued with his premise that officeholders face an excruciating financial burden in keeping two homes, buying lunch, drinks, flowers and the like.

"The whole point is [that] not all congressmen are rich and have big houses in Chevy Chase," a Rostenkowski aide said.

State legislators are allowed to deduct $50 a day for business expenses, but Congress has been unwilling or afraid to give itself the same break.

The Joint Committee on Taxation estimated that Rostenkowski's proposal would have cost the Treasury $2.8 million in lost taxes. Another plan, co-sponsored by 17 congressmen, would raise the deduction to $6,000 a year and would cost the Treasury an estimated $800,000 annually.

Still a third plan, with no particular champion, would simply adjust the 1952 figure to the rate of inflation, giving members deductions of about $10,200 and costing the Treasury an estimated $1.8 million annually.

Members apparently would fare best under Whitten's proposal to eliminate the $3,000 ceiling. Studies by the Joint Committee find this would allow members to deduct rent, meals, transportation, laundry and parking expenses.

Members who own homes in this area would be eligible for depreciation allowances and deductions for such things as maids and gardeners, if they could prove these were "ordinary and necessary business expenses," a committee source said.