If President Reagan gets his way, next year's budget won't preserve much funding for historic preservation.

Under the president's proposals for the fiscal year that begins in October:

* The bulk of the program, $21 million worth of grants to state historic-preservation offices, would be eliminated. The offices determine whether properties belong on the National Register of Historic Places, whether proposed federal projects will unnecessarily damage historic sites, and whether rehabilitation of historic properties qualifies for a tax credit that took effect last year.

* The entire $4.4 million federal grant to the National Trust for Historic Preservation, which operates historic properties and encourages preservation, would be cut, eliminating 40 percent of the trust's budget.

* The $1.6 million that the government's Advisory Council on Historic Preservation now gets would be reduced to $1 million, and its staff cut from 40 to 25.

On the other hand, the preservation budget of the Interior Department's National Park Service would grow slightly--by $800,000 to $4.4 million. The extra money would be used to administer the National Register and the tax incentive.

But preservation groups maintain that the increase will not make up for the grants to the states, which now do most of that work.

Nellie Longsworth, president of the lobbying group Preservation Action, described the Reagan move this way:

"What he's saying is that what they spent $21 million on last year they're going to spend $800,000 on this year."

Because the states now perform many activities that are required by federal law, cutting their grants will end up costing the federal government more than it would save, Longsworth and others contend.

"In our view the federal government's getting a bargain," said Michael Ainslie of the National Trust, noting that states must now match at least half their federal grants. "States are implementing federally initiated responsibilities and they're paying for half of it."

While Reagan's "New Federalism" policies are designed to shift responsibilities from the federal government to the states, preservation groups argue that the funding cuts will have the opposite effect.

Historic preservation "is a model for the way the New Federalism ought to operate," J. Rodney Little, president of the National Conference of State Historic Preservation Officers, told a House appropriations subcommittee last March. "Certainly we shouldn't be one of the victims of the New Federalism."

Indeed, the government's own studies support the view that the states do the job for less.

A report by the Advisory Council on Historic Preservation estimated that it would cost the federal government at least twice as much to do the states' work.

And a study by the Interior Department projected that the cuts would increase federal costs next year by as much as 18 percent and require a staff increase of up to tenfold--even assuming a drop in services of up to 35 percent.

"While federal agencies would be required to live up to the law, the question seems to be how they are going to do it," Little said. "On paper they would have the responsibility, but in actuality that would not be happening."

Interior Secretary James G. Watt contends that states and the private sector will supply their own funds to fill in the gap and has disavowed the department study.

But preservation officials dispute Watt's prediction. If anything, they say, states will follow the federal lead and slash their own funds as well, especially in light of the increased strains on their budgets caused by other federal cuts.

"A fact of life the Reagan administration doesn't seem to recognize is that, historically, when the federal government pulls back, state governments do not step into the void," said Little, who heads Maryland's historic-preservation office.

The void may be felt particularly in the tax incentive, which gives a 25 percent tax credit to developers who renovate properties listed on the National Register of Historic Places. The Park Service estimates that $922.5 million worth of construction will be judged eligible this year.

Preservation officials and lobbyists argue that Watt, who supported the tax credit, is now undermining it by not giving it enough money to be administered.

States not only helped certify that proposed rehabilitation projects met the proper standards, Longsworth noted, but were deluged by nominations to the National Register, whose listings have increased by 80 percent since the first historic-preservation tax incentives took effect in 1976.

Without state staff to do that work, she warned, the tax credits will be meaningless. "If you're looking at six months or a year delay in certification, a developer isn't going to wait because he can do other things with the money," Longsworth said.

Certification "will come to a screeching, murderous halt" if the cuts are enacted, Little predicted, saying states now put in 9 1/2 hours for every one hour of federal time on the program.

Preservation officials and groups say they are already feeling the impact of earlier cuts, which reduced funds for states and private groups from a high of $60 million in 1979 to $25.4 million this year.

The administration had tried to eliminate the grant program last year--except for $4.4 million to the National Trust--but Congress stepped in and restored funding.

This time, however, the budget is even tighter, and it is unclear whether Congress will repeat its rescue.

The lobby groups are hoping to maintain funding at current levels. If the cuts go through, Little warned, "it will mean an awful lot of damage to historic properties that presently is averted by the system."