The combined impact of the 1981 tax law and the federal government's curtailment of new subsidized housing has left those in the business of selling real estate tax shelters scrambling for well-maintained older apartment buildings and properties designated as historic.

The real estate tax shelter industry continues to grow, but the segment that deals with federally financed housing is running out of its traditional stock-in-trade, construction of new subsidized projects. The last of these are now trickling out of the pipeline, forcing the syndicators to try refinancing well-kept older federal projects or to switch to conventionally financed apartments and office buildings.

"The decline in federal financing, coupled with the beneficial tax changes, has increased competition among sponsors the tax shelter sellers and should result in a broader variety of tax-oriented real estate investments becoming available to the general public," said Ken Michel, executive vice-president of Langelier Company, Inc., a Boston-firm that specializes in real estate tax investments.

The nation's income-tax laws have always allowed owners of rental properties, including subsidized and unsubsidized apartment projects, office buildings and shopping centers a variety of tax breaks,, but those allowed the owners of federally-assisted housing are the most favorable in order to attract private investment to such projects.

The tax benefits generated from rental properties are so large that developers often sell a share in the ownership of the property to high-income investors, entitling them to a portion of the write-offs. These deductions allow doctors, lawyers and corporate officers to shelter a portion of their salaries and unearned income.

Investor groups are matched with developers by firms specializing in real estate tax shelters. These firms are called "syndicators" and the sales to investor groups is technically called "syndication" of the tax losses.

The Economic Tax Recovery Tax Act of 1981 boosted these real estate tax shelters in two ways--it shortened the time period for writing off the cost of buildings (depreciation) thus increasing these deductions. In addition the more favorable depreciation schedule previously available to only new buildings was extended to old ones as well.

Federally assisted housing, which has always enjoyed higher write-offs than conventionally-financed apartments, has become even more valuable with the tax changes and now the scarcity of new units.

But the tax act also hurts tax shelters. The law lowered the maximum tax rate on unearned income from 70 to 50 percent thus making the write-offs less valuable to high income individuals. Whereas before, a dollar deducted could protect as much as 70 cents of income from interest, stocks, real estate or the like, today the same dollar shelters only 50 cents.

At the same time that the Congress was changing the tax laws to increase the real estate tax deductions, the Reagan administration began taking the federal government out of the business of building new subsidized housing for the first time in 40 years.

According to Robert Stanger, whose newsletter follows the tax shelter industry, in 1981 $700 million was raised and invested in newly built federally-subsidized housing. Last year the figure had fallen to $500 million and "it will be virtually non-existent by 1984," Stanger said.

Investments in real estate tax shelters in general grew from $3 billion in 1980 to $4.7 billion last year, Stanger said.

Federally subsidized housing, with its larger write-offs, has been favored by high income investors but with the tax reduction on unearned income, Stanger and others said investors are turning to properties offering not only protection of incomes but also a good potential for profits from rents and the building's sale once the tax benefits are exhausted.

"There is a sharper look at shelters when people are saving less in taxes," said David Reznick, a local accountant.

Firms dealing primarily with selling tax shelters in federally subsidized housing, faced with both the tax changes and the federal cutbacks, are shifting their emphasis from new to older federally financed housing and broadening their portfolios to include non-subsidized apartment buildings and offices and historically-designated properties.

The new tax law allows owners who restore historic buildings to take as a tax credit 25 percent of the rehabilitation costs. Previously the tax credit had been only 10 percent.

"With the end of the federal subsidies, people involved in syndicating subsidized housing are devoting their energies to re-syndications," said Robert Tracy, executive vice-president of the National Housing Partnership, one of the largest syndicators of federally-assisted housing.

William Dockser, head of CRI, Inc., a local real estate syndiciation firm specializing in federal housing, echoed many in the industry when he said, "We're looking for subsidized projects that are 10 to 12 years old with eight years to go on their 20-year contracts that could be turned into condominiums."

The syndicators want the well-maintained suburban projects that need relatively little money for improvements and where all the tenants regularly pay their rents.

The federal government only requires owners of subsidized housing to rent to poorer families for 20 years. After that an owner can pay off the remaining mortgage on the building and begin charging market rents or convert the building to condominiums and evict lower income tenants.

Federal officials and owners of federally subsidized projects are overjoyed with the new interest in their properties. It means that the current owners of these projects who thought they would have to hold onto them for 17 years while their tax deductions are dwindling (because most are taken in the first five years of ownership) are able to get their money out earlier.

HUD officials are happy because typically the current owners have little money to spend on sprucing up the buildings. HUD officials say they require new owners to make all needed repairs.

Federal officials report that during the past year roughly 64,000 units or 20 percent of the nation's early stocks of subsidized housings have changed hands.

"This is a good program if handled properly," said Maurice Barksdale, HUD deputy assistant secretary for multi-family housing. "It provides an infusion of money to get units back in shape and to keep the project solvent." He added that the non-profit groups who previously owned the projects) "don't have the money for these long-term repairs."

The deals are structured so that there are no rent increases, which would required federal approvals, experts said. In a typical deal a syndication company goes out and finds a number of higher income professionals who want to contribute cash to become limited partners in a series of federally-assisted properties.

The cash is then used as a downpayment, the new group of owners assume the existing HUD mortgage and the seller takes back a second trust. HUD requires that the total amount of the mortgages not exceed the market value of the project.

Charles Edson, a local attorney specializing in housing finance said the syndications of older subsidized apartment projects "helps to keep the buildings in good condition and that is very important since we are not putting up new buildings."