In proposing additional "12 percent cuts" in all government programs other than defense and entitlements such as Social Security, President Reagan has broken most of the deals he made on Capitol Hill to win passage of his budget proposals last spring and summer. He also has asked for real cuts that will go much deeper than 12 percent for some big programs.

When the president spoke last night of 12 percent reductions, he did not spell out that he meant 12 percent below his original budget-cutting proposals of last March. Since then, the administration has accepted numerous changes in those proposals, including some that markedly increased spending for education, Amtrak and Conrail, public housing, low-income energy assistance, student loans, highways and others.

Those changes were made to win votes for the overall budget ceilings Reagan sought. Reneging on them now would mean cutting Amtrak, for example, not by 12 percent but by 46 percent--or $735 million vs. $394 million-- from the level of spending approved in the reconciliation bill passed July 31.

Title I education assistance to needy children would be cut 29 percent below the reconciliation level, or from $3.48 billion to $2.475 billion.

A senior administration official who briefed reporters last night on the new budget-cutting plans said "we didn't have much choice" about reneging on deals made in Congress since last March, because of unforeseen economic developments.

He also said, "There was never any commitment on our part to seek funding" at ceilings authorized in the reconciliation--a statement that many members of Congress will read with surprise.

Cuts included by Reagan under the "12 percent" category would amount to half of the $16 billion package he outlined last night. According to the official who briefed reporters, all but a handful of the thousands of individual government spending accounts would be affected.

Exceptions listed by the White House last night were the Immigration and Naturalization Service, direct Veterans Administration hospital care and peacekeeping forces overseas. Others will receive partial exemptions, and a full list will be issued later.

Government programs whose benefits are guaranteed by law to the needy--Social Security, Medicare, food stamps and other so-called entitlement programs--are excluded from these cuts but will be subject to later cuts.

The Northeast-Midwest Congressional Coalition, which represents the "gypsy moth" states, last night made available calculations showing the difference between 12 percent cuts based on the president's March proposals and the spending levels Congress thought it had finally approved in the much-disputed reconciliation bill.

Among those differences:

* Aid to low-income families for energy costs, for which Congress voted $1.875 billion. Under the president's new proposal, $1.232 billion would be spent, a cut of 34 percent.

* Education aid for the handicapped, for which Congress voted $1.15 billion. Reagan now proposes $783 million, a 29 percent cut.

* Guaranteed student loans, for which Congress voted $2.753 billion. Reagan proposes $1.561, a 43 percent cut.

* Conrail, the Northeast's freight train network, for which Congress voted $386 million. Reagan proposes $44 million, an 89 percent cut.

* Federal highway programs, for which Congress voted $8.2 billion. Reagan proposes $7.04 billion, a 14 percent cut.

* "Impact aid" to school districts that educate the children of government employes, for which Congress voted $475 million. Reagan proposes $353 million, a 26 percent cut that would hit the Washington area's school districts particularly hard.

* Economic Development Assistance for local public works projects, for which Congress approved $265 million. Reagan proposes $28 million, an 89 percent cut. Two programs Congress thought it had approved--low income weatherization assistance ($175 million) and the solar and energy conservation bank ($50 million)--would be wiped out by the administration proposal.

In other programs, Congress went along with funding levels Reagan proposed last March, so the 12 percent cut for them would be an actual 12 percent. Among them are bilingual education grants, Northeast rail corridor improvements and some of the CETA programs.

The official who briefed reporters last night said the administration's plans for cutting the big entitlement programs would emerge after further study. Next week, the administration will convene an interagency task force to complete work on a concrete legislative proposal.

Apart from Social Security, which the administration wants a bipartisan commission to review and propose changes for, the White House said last night that every entitlement program in the government would be reviewed. The official mentioned Medicare and Medicaid, food stamps and the federal employes retirement program as entitlements he thought should be modified to reduce their cost.

The administration plan calls for $2.6 billion in unspecified savings from the entitlement programs in the fiscal year beginning nextThursday, all of which would have to be approved by Congress.

The White House also plans to reduce pressure on the nation's financial markets by cutting by about one-fourth the amount of government-guaranteed loans to be issued in the new fiscal year.

Although these securities--for example, money raised by the Rural Electrification Administration, the Export-Import Bank, the Small Business Administration and others--do not appear on the federal budget, they compete for private capital just like Treasury bills that finance the current federal deficit.

Losing some of this money would have a major effect on the agencies and programs involved, even though it would not improve the overall budget picture.

Researcher Maralee Schwartz contributed to this article.