Proposed cuts in Social Security benefits reflect "what amounts to an agreement by both Congress and the administration to lower the safety net" on key spending programs the president pledged to leave intact, a senior Reagan official said yesterday.

Lawrence Kudlow, chief economist at the Office of Management and Budget, told reporters yesterday that the drastic changes in Social Security proposed by the administration earlier this week "will continue to the additional budget savings" needed to meet Reagan's budget projections in later years.

Kudlow also predicted a sharp fall in interest rates in coming months. He acknowledged that skepticism in financial markets about the administration's ability to cut federal spending and inflation had pushed interest rates up, while the Reagan forecast assumed a rate fall.

But the Social Security proposals mean that "as we sit here the 1982 budgets is changing" and the cuts the administration has proposed in nondefense spending "are getting deeper," Kudlow said.

He said that he believes that the official interest rate forecast for an average rate of 11 percent on three-month Treasury bills during 1981 still could be met. As rates are now over 16 percent, this would require a very steep fall in later months. Kudlow said that was a "long process of gradually proving to the markets that government has changed." But a sharp fall in rates in the third quarter is likely, he told reporters.

Kudlow took issue with recent remarks by Wall Street economist Henry Kaufman that the Reagan program would be inflationary because of large rises in defense spending and cuts in taxes. He said the proposed defense build-up still would leave overall spending falling because of sharp cuts in nondefense spending.

Hinting that defense spending itself may be trimmed, Kudlow said that the administration still is searching for budget savings, adding that "nothing is untouchable, including defense." He said that defense spending had not yet begun to be scutinized for savings, but that it would be.

The administration already is faced with budget over-runs for the current fiscal year which ends in September. Kudlow said that OMB is "doing what we can to find ways of cutting back on the overrun, which could take the 1981 deficit to more than $60 billion, instead of the $54.9 billion in the administration's budget projections. However, another senior Reagan official said earlier this week that the administration may have to live with the excess.

Concern in the financial markets about the difficulties the administration faces in curbing the deficit is a crucial factor behind recent interest-rate rises.