The House Ways and Means Committee agreed yesterday to modest cutbacks in the tax credit for rehabilitation of aging buildings, which supporters say has been responsible for the revitalization of aging downtown areas in Washington and elsewhere.

The limitations, voted as part of the committee's massive tax-overhaul bill, would retain more than half of the credit's value. The credit is one of the few tax provisions committee Chairman Dan Rostenkowski (D-Ill.) has endorsed as the tax code is up for examination, and preservation groups said they could live with the reduction.

"It sits well with us," said Ian Spatz, legislative counsel for the National Trust for Historic Preservation. "We're going to see what happens with the rest of the bill, but it's very gratifying to see the committee turn around the president's recommendation to repeal the credit."

President Reagan had proposed wiping out the credit entirely in his tax-revision package, on the grounds that it was not an effective way of encouraging rehab and did not stimulate revitalization of deteriorating areas as much as it subsidized downtown renovations that would have taken place anyway.

The cutbacks voted by the committee would reduce the credit, which now can cut taxes by 15 percent to 25 percent, depending on the age of the building and its historic value, to either 10 percent or 20 percent. The curtailment would raise $2.6 billion in tax revenue, while Reagan's repeal would have raised $6.7 billion from 1986 to 1990.

According to figures compiled by the National Trust, the 25 percent rehabilitation credit was used in 145 projects in the District between 1982 and 1984, worth a total of $200.4 million. Nationwide, it has covered about $5 billion worth of rehabilitation projects.

The credit will subsidize an estimated $10 million of the $108 million it will cost to renovate the Willard Hotel, and also was used in the rehabilitations of the Old Post Office and other District projects.

The committee's action was one of several taken in the tax-credit area yesterday, and it came during a day in which Rostenkowski got his way on a number of amendments. As Ways and Means has abandoned the practice of roll-call votes, which are released to the public even though the meetings are closed, more members have begun voting to curtail popular deductions in order to pay for reducing tax rates.

Ways and Means is to meet today on taxation of companies' foreign operations, and has scheduled a rare Sunday session as well. Rostenkowski hopes to complete tax action next week.

The committee voted to let a number of tax credits expire, as they are scheduled to do at the end of the year. Among them: the 15 percent credit for energy-efficient home improvements and the 40 percent credit for wind and geothermal installations.

Panel members accepted an amendment by Rep. Wyche Fowler (D-Ga.) to phase out the 40-percent credit for solar expenses by individuals and 15 percent for expenditures by business over three years. And they approved a similar proposal by Rep. Cecil Heftel (D-Hawaii) to extend the 15 percent business credit for geothermal energy for three years. Members also agreed to phase out a credit for alcohol fuels, but decided to retain a 6-cents-per-gallon gasoline-tax exemption for users of gasohol.

Earlier, the committee accepted the recommendation of a task force and agreed to remove the tax exemption for Blue Cross and Blue Shield health insurance plans, thus taxing them the same way competing insurance firms in the private market are taxed. Insurance plans provided by fraternal organizations, such as the Knights of Columbus, would be exempted from taxation.

A third category of health insurance coverage, provided for some 1 million employes of colleges and universities, not only would lose its tax exemption inder the committee action, but also could find its pension plans taxed as well.

In voting to tax insurance provided by an organization known by the acronyms TIAA/CREF, Ways and Means included earnings from that organization's pension plan. If that became law, earnings from the pension fund's investments would be taxable income to the fund, while such earnings are not taxed in other traditional retirement programs.

Ways and Means aides pointed out that the TIAA/CREF plans could avoid this taxation by separating the pension monies from the health-insurance fund, something insurance firms routinely do.