The Carter administration yesterday notified the House that it would go along with building a conventionally powered aircraft carrier, a policy change that threatens to undercut a legislative effort to save the president's defense bill.

Chairman Melvin Price (D-Ill.) of the House Armed Services Committee said as debate opened on the Pentagon money bill that the administration, by agreeing to build a conventionally powered aircraft carrier for $1.5 billion, had narrowed the difference between the committee and administration defense bills to $800 million.

The committee recommended authorizing $37.9 billion to buy weaponry and finance research in fiscal 1979, $2.4 billion more than Carter requested. Rep. M. Robert Carr (D-Mich.) contends that most of the extra millions are "pork-barrel" additions which should be deleted by passing a substitute bill conforming more closely to the president's defense program.

Carr, who said he intends to offer his substitute on the House floor today, said that the administration's expressed willingness to spend $1.5 billion on a conventional carrier after saying it wanted no money at all this year for any kind of carrier "has muddied the waters." He said this muddying will undercut the effort by him and his allies to restore the president's defense program.

"The administration clearly doesn't have its act together, and I'm angry about it," Carr said last night. He said that William Cable of the White House legislative liaison staff and Edward R. Jayne of the Office of Management and Budget were among those officials who told him the administration supported his substitute bill.