Washington area governments could face a 5 percent reduction in new federally subsidized housing units this year and a 22 percent reduction next year compared to 1980, if President Reagan's plans to curtail federal spending are approved.

Those are the findings of the Washington Planning and Housing Association, a private, nonprofit group that advocates more federally subsidized housing. The group's report shows that area jurisdictions from D.C. to Falls Church to Greenbelt will receive federal dollars for nearly 200 fewer subsidized units this year and nearly 800 fewer in fiscal 1982 compared to 1980 under the Reagan proposals now before the Congress.

The report is the first to translate the proposed budget changes into reduced subsidized housing for low- and moderate-income families, which for a family of four is defined as having an income less than $21,600.

"You're knocking nearly 1,000 families out and just forgetting about them," said James Harvey, the executive director of the association.

Samuel R. Pierce Jr., secretary of the U.S. Department of Housing and Urban Development, in announcing the budget changes last month, said they were "absolutely necessary" in the administration's battle "against inflation and to restore the nation's economic stability." Pierce said "it is our goal to eliminate those programs which are inefficient or which overlap other programs, and to improve the efficiency and effectiveness of the worthwhile programs.

Area housing officials said the reductions come at the very time that low- and moderate-income families already have great difficulty finding affordable apartments and homes in the District and its suburbs. They are trapped between the growing popularity of converting apartments to condominiums, which results in the displacement of families who cannot afford to buy, and high interest rates that make it difficult for developers to build new apartments for anyone but the rich.

"We're in the tightest housing market we've had for a long time," said Bernard Tetreault, executive director of Montgomery County's Housing Opportunities Commission.

There are 7,000 families waiting for subsidized housing in the District, 6,000 in Montgomery, 1,500 in Prince George's and 1,800 in Fairfax, said housing officials in these jurisdictions.

In fiscal 1980, the Washington area received 3,710 subsidized housing units under the federally funded public housing program, in which renters live in publicly owned housing, and the Section 8 program, in which the federal government pays a portion of the rent for those living in privately owned housing that meets federal guidelines.

The area would have received an estimated 4,391 new units this fiscal year and 4,318 in fiscal 1982 under the old President Carter budget. Under the Reagan proposal to change that budget, the area would receive only an estimated 3,521 new units this fiscal year and 2,926 units in fiscal year 1982. e

"I think the cuts will be even more severe," said Robert L. Moore, the District's housing director. "The Senate budget committee cut even deeper than what Reagan wanted." On March 23 the committee, with a Republican majority, reduced the number of subsidized units available nationwide in fiscal year 1982 to 150,000 -- 25,000 less than the recommended Reagan reduction.

And there's a sleeper in the budget for the city, said Moore. The new administration wants to spend a larger percentage of its housing money subsidizing the rents of families who find housing in existing privately owned apartments instead of financing the construction of new apartments, which is more costly. This is the opposite of the policy adopted by the Carter administration, which put more emphasis on building new subsidized housing.

This is bad news for much of the Washington area, said housing officials, because of the low 3.6 percent metropolitan vacancy rate for apartments, a shortage of apartments in general and a dearth of low-rent apartments. That will make it more difficult for subsidized tenants to find housing in existing privately held housing.

The change in emphasis from new construction to existing housing, however, may benefit Prince George's County. The jurisdiction places a large number of low- and moderate-income families in existing privately owned apartments, because the county has a vacancy rate of about 6 percent, said Earl Morgan, director of Prince George's housing authority.