A Senate subcommittee approved a slimmed-down plan yesterday to restore the U.S. Capitol's crumbling West Front, amid indications that support for a rival House-supported measure to extend the building instead may be eroding like its sandstone walls.

The Senate plan pares from $66 million to $48 million estimates from the Architect of the Capitol for restoration of the West Front, and, for the first time, calls for a fixed price contract rather than the open-ended cost-plus-profit contracts that have ballooned the price of government buildings in the past.

It also sets the stage for another House-Senate battle over the debate to extend or restore, which has snarled both houses of Congress sporadically for more than a quarter of a century.

The House Appropriations Committee voted Wednesday to spend $70.5 million to extend the West Front about 31 feet toward the Mall.

But the action came only after the committee startled supporters on both sides of the issue by, in effect, scrapping long-drawn detailed plans that would have stretched it six feet farther.

Architect of the Capitol George White acknowledged in a letter to Rep. Vic Fazio (D-Calif.) that through an "inadvertent oversight" the six-year-old expansion plans would reduce the marble terraces overlooking the Mall, while providing 79,000 square feet of new office space and hideaways for congressmen.

Fazio, on a voice vote, added "noncontroversial technical language" to correct that error, reducing the extension by six feet. Committee staff members said, however, that the correction will require extensive revisions in detailed architectural drawings, debated for six years, plus new cost estimates that could further weaken House support for the measure.

"It's a whole new ball game," said Sen. Alfonse M. D'Amato (R-N.Y.), chairman of the Senate legislative appropriations subcommittee that voted yesterday for restoration instead of expansion. "If you believe their $70.5 million figure now I want to sell you some swampland in New Jersey."

House supporters of the extension say it is needed to provide additional House office space, particularly for the cramped Appropriations Committee. Opponents, which include the American Institute of Architects and the National Trust for Historic Preservation, argue that the West Front should be preserved and any additional needed space found in other buildings.

The West Front is the only remaining original wall of the Capitol. The sandstone facade has been crumbling for decades, and last month a 16-foot section fell away, spurring demands for prompt action of some sort to keep the building from further deterioration.

The Senate measure approved yesterday provides for about $17 million in actual construction costs in the restoration. The remainder would cover 10 percent inflation during the estimated 40 months of construction, 20 percent profit for contractors, 5 percent for subcontractors and a 25 percent contingency fee for unknown factors encountered behind the crumbling sandstone.

Staff members say the plan probably will be reported to the full Senate Appropriations Committee at 10 a.m. Thursday.