The number of federally subsidized low- and moderate-priced rental units in the United States could drop by two-thirds over the next 12 years because of expiring federal housing restrictions unless the Department of Housing and Urban Development and Congress act to halt the decline, according to a new study of the nation's rental housing stock.

Altogether, more than 1.8 million units of privately owned, federally subsidized low- and moderate-income housing out of a total of 2.9 million such units across the country could be lost by the year 2000, the National Housing Preservation Task Force, a coalition of real estate trade group representatives, reported in the study. It was given to members of the Senate Housing and Urban Affairs subcommittee earlier this month.

The findings come at a time of mounting concern about a lack of affordable housing in the Washington area and nationwide. A total of 13,000 people are waiting for low-to-moderately priced housing in the District, while there is a waiting list of more than 5,000 in Montgomery County and 2,700 in Fairfax County. Rising home prices are pushing more low- and moderate-income residents into counties farther beyond the metropolitan area than before or into shelters for the homeless, local housing officials said at a recent meeting on housing problems.

The task force's report primarily blamed the expiration of federal rental subsidies for the impending decline in the low- and moderate-priced housing stock. It estimated that as many as 2.2 million low-income units are in danger of being phased out as the terms for various programs expire and landlords decide not to renew their contracts to keep such housing available and instead offer the units at market rates.

Landlords have already agreed to keep rents low for 280,000 units, in exchange for a direct subsidy from HUD. The subsidies initially were good for 15-year periods and could be extended for another 15 in five-year increments. The program began in the early 1970s, but HUD has been reluctant to create more long-term agreements with landlords. Instead, the agency has been giving only five-year contracts for the rent subsidies, which will allow many to expire and let landlords start charging higher rents, the report said.

The report also said the possibility of owners prepaying federally subsidized mortgages is also a factor in the anticipated drop in the number of low-cost units. According to HUD, more than 360,000 units in 3,215 complexes were financed under two programs that either provided landlords with low-interest mortgages or reduced mortgage rates to 1 percent, enabling complexes to charge low rents. But these programs, which went into effect in 1969, allow owners to pay off their mortgages after 20 years, boost rents or convert their projects to condominiums. The task force estimated that many of the owners of 154,279 such units would opt to do so when their mortgages reach the 20-year mark over the next decade.

The report also estimated that a significant number of units will fall into disrepair and become uninhabitable, thus further shrinking the supply of affordable privately owned subsidized housing.

To stem the loss of low- and moderately-priced rental units, the task force called on HUD to extend the life of subsidies -- in the case of complexes with federally subsidized mortgages -- to the full term of the loan. The report also recommended that more public assistance contracts be set up with landlords to guarantee that units are set aside for low-income housing.

Moreover, the report urged Congress and HUD to offer incentives to owners of federally financed projects to forgo paying off their mortgages. The task force said one way to do this would be to allow owners to receive a larger yearly return on their investments. Landlords currently can make a profit of 6 percent a year based on their initial investment; any excess profits go to HUD. Instead, the report said Congress should allow owners to calculate their rate of return based on current market values, which would allow them to earn more of a profit.

In addition, owners should be allowed to take a second mortgage on a project's equity so they don't have to pay off a mortgage and sell a property to obtain the cash value from it, the report said. It urged Congress to pass new tax provisions allowing owners to depreciate the value of their complexes a second time once they have already used up their original depreciation schedule.

To help maintain current units, the task force recommended that HUD establish a revolving loan fund out of which landlords could borrow money to renovate projects at interest rates that would not exceed 6 percent.

If owners used any of these programs, they would be prohibited from prepaying their mortgages and would have to adhere to federal rent guidelines, the task force said