President Carter, under pressure from the timber industry, yesterday told the Interior and Agriculture departments to increase significantly the timber harvest in national forests.

The move, an unprecendented departure from the historic policy of cutting only as many as can be replanted, is "an important contribution to easing the rising cost of housing," White House inflation adviser Alfred Kahn said.

The new policy is expected to increase cutting in the national forests by one billion to three billion board feet a year within the next two years. Private companies now harvest 12.4 billion board feet a year from public lands.

The skyrocketing cost of housing has been one of the most stubborn contributors to inflation. Wood products account for about 15 percent of the cost of an average house.

A timber harvest increase of a billion board feet would cut the cost of a new house by six-tenths of a percent, administration figures show. The median price of a new home rose 120 percent from 1971 to 1978, while overall consumer prices increased about 60 percent.

The new policy, billed as "a limited and temporary departure from the nondeclining even-flow policy," would allow the government to sell "excessive inventories of mature timber . . . on a number of western national forests, much of which would be wasted" under current rules, Kahn said.

Environmentalists, who successfully sued to stop President Nixon from abandoning the "even-flow" timber policy in 1974, called Carter's action "a giveaway" to the timber industry. "Ever since the establishment of the national forests we've had a historic policy that you don't cut more than you grow," said William Turnage of the Wilderness Society.

Thomas Barlow of the Natural Resources Defense Council said, "Carter has firmly opened the door to a massive overcutting of the national forests in the West . . . Almost a billion board feet a year are going to waste and dying in private forests because the government is already selling too many trees from national forests."

However, Eugene Bergotten of the National Forest Products Association praised the administration for taking "the first tangible step toward productivity . . . "We've got to get the trees down before they die," he said. Bergoffen predicted the move could affect the prices companies bid for timber "within several months or a year."

"It will dampen the inflation rate," he said.

Kahn emphasized that the action will be "consistent with existing law and environmental considerations." He said the temporary departure from past policy "will not cause an eventual decline in the long-term flow of timber harvested from federal land; in fact, it will probably mean a higher rate of growth and production from these lands in the future."

However, Forest Service Chief John McGuire, acknowledged that as a result of cutting more timber now, "At some point in the future, we would have to cut less. Then we could go back to a sustainable yield. It's a balancing act."

The new policy has been the subject of months of interagency fighting, with the Council on Environmental Quality and the Forest Service reportedly opposing the Council on Wage and Price Stability's plan to increase cutting.

"There's serious question as to the legality of increasing the cut," one opponent said. "We shouldn't politicize the national forests, especially when it doesn't do anything for the inflation rate."

Timber companies have been eager to increase cutting on federal lands, partly because they have heavily cut their own land in recent years. If timber production is to remain stable, they argue, the government should compensate for the shortfall on private land.

They have been especially disturbed that the Office of Management and Budget vetoed $77 million the Forest Service requested to plan for the sale of an additional 750 million board feet this year. The maximum sustainable yield from federal land is about 15 billion board feet, McQuire said, but much of that timber would be unsuitable for housing.

Agriculture Secretary Bob Bergland said the Carter decision "is likely to be interprted as anti-environmental, but it is [not] . . . It is a carefully balanced approach." He said he has received equal pressure from both the timber industry and from environmentalists.