The use of tax-exempt bonds to assist first-time home buyers is an expensive and inefficient form of subsidy, and the majority of the beneficiaries are people "who probably could have purchased homes without assistance," according to a study by the General Accounting Office.

The bond mechanism is so inefficient that the same level of help could have been provided for roughly a quarter of the cost through a straight subsidy program, the GAO said.

The congressional agency calculated that approximately $10 billion was raised with the bonds in 1981 and 1982, which will result in a tax revenue loss of $2.66 billion. "A direct subsidy program providing the same number of loans could have been funded for about $680 million--a savings of about $2 billion," the report said.

"Providing subsidies directly to households using a grant or carefully structured tax credit would be less costly than mortgage revenue bond financing," the study concluded.

The federal law providing authority for the bonds--the 1980 Mortgage Subsidy Bond Tax Act--expires at the end of this year, and states and localities are campaigning energetically to have it extended. Thomas W. White of the Council of State Housing Agencies told a Senate Banking subcommittee last month that elimination of this "sunset" is his group's "number one priority."

Proponents of the bonds point to their widespread popularity and the continuing problem that first-time buyers have coping with both the high cost of houses and the high cost of money.

"Last year one-third of all first-time home buyers in this country received financing through the use of mortgage revenue bonds," according to White.

Opponents, who will gain considerable ammunition from the GAO report, long have argued that the bonds are unnecessary and a drain on the Treasury.

The report was requested by Sen. Robert J. Dole (R-Kan.), chairman of the Senate Finance Committee.

The subsidy works this way: A state agency issues tax-exempt bonds. Because of the tax benefit, these bonds carry below-market interest rates. The agency then takes the money it has raised and makes low-interest loans to home buyers.

The problem, the GAO report said, is that "the loss in federal revenue--the largest single cost of mortgage revenue bonds--is inevitably much greater than either the reduction in borrowing costs to state and local governments or the reduction in interest rates to home buyers." And it noted that "the average buyer of tax-exempt mortgage revenue bonds, who is typically a high-income individual or financial institution, receives tax savings much greater than the interest savings provided to the average assisted homebuyer."

The report also "found that most subsidized home loans were not made to low- and moderate-income households in need of assistance, but rather to those who probably could have purchased homes without assistance." Indeed, the "typical" home buyer under the program last year was "an individual or two persons between 20 and 35 years of age with an income between $20,000 and $40,000," the report said.

The GAO also found that "53 percent of the subsidized borrowers were among the more affluent half of the families in their states."

Only about 25 percent of mortgage revenue bond funds went to low- and moderate income households--those with less than 80 percent of the median income for their area--it said.

The fact that most of the aid goes to higher-income people means that the cost-effectiveness of the program is "further degraded," GAO said. If one assumes that the program is meant to aid low- and moderate-income people but only a quarter of the aid actually gets to them, "then the actual cost per targeted household would be four times the cost we estimate in this report," GAO said.

Finally, the GAO noted, "The issuance of mortgage revenue bonds has been found to have a negative impact on interest rates for other state and municipal borrowing." This is because the growing popularity of the program has resulted in large numbers of offerings, resulting in an oversupply of tax-exempts on the market, which in turn means that interest rates have to rise to attract buyers.