Residential property-tax assessments in Washington dropped four-tenths of 1 percent over the last year--the first decrease in at least 20 years--reflecting the effects of recession and high interest rates on the city's real estate market.

At the same time, commercial property assessments increased by 7.1 percent, or about one-third the increase recorded the previous year. Overall, the city's real estate assessments have gone up a scant 2.2 percent, compared to a 14.9 percent increase in 1982, according to new D.C. government figures expected to be formally released today.

Alphonse G. Hill, deputy mayor for finance, said yesterday the figures mean the city may have to approve a small tax-rate increase in order to balance the 1984 budget approved by the City Council last week.

"If there's any increase in the tax rate , maybe it will be a penny or two or three, but nothing significant," Hill said. "We'll just have to control our expenditures if our revenues don't hold up."

The city needs to raise $362 million in property-tax revenues to balance its $1.9 billion budget. City planners had estimated that that amount would be financed by a 3.1 percent increase in property values--slightly more than the actual increase of 2.2 percent.

The D.C. Department of Finance and Revenue is expected to formally release the new figures today after mailing 1984 real estate-assessment notices to about 153,000 property owners.

The new property assessments are subject to appeal before the D.C. Board of Equalization and Review. By law, Mayor Marion Barry must certify the new property-tax base to the City Council by July 1. The council then has 20 days in which to set the new tax rates.

The D.C. government reassesses property every year based on a variety of factors, including actual sales, the estimated cost of replacement less depreciation, and the amount of rental income that the property generates.

The drop in the assessed value of owner-occupied residential property this year is in sharp contrast to real estate trends in the District over the past decade. Assessments increased steadily from tax year 1974 to 1983, with growth averaging 21 percent a year between tax years 1977 and 1982 for single family homes.

The rate of increase for owner-occupied residential property moderated to 9.4 percent for the 1983 tax year, but an actual decline in assessed values has not occurred since the beginning of the city's assessment records, which date back to the early 1960s.

City officials and real estate experts yesterday blamed the lackluster real estate market on high interest rates, increased unemployment and a generally gloomy economy.

"The most important factor was high interest rates, which made sales decline," said James G. Banks, executive vice president of the Washington Board of Realtors. "People weren't willing to pay those high interest rates . . . The economic atmosphere was frightening to people and they weren't willing to make long-term commitments."

Banks added that he was a little surprised by the 7.1 percent increase in the assessed value of commercial property, in light of a substantial decline in demand. "I didn't think they'd be up that much," he said.

The assessment on the average single-family, owner-occupied home decreased from $90,489 in the current 1983 tax year to $90,179. Taxes on such a home would total $990 a year, assuming no change in the current tax rate.

Some neighborhoods showed far greater than average decreases in residential property valuation, including Barry Farms, down 19.3 percent; Burleith, down 11.6 percent; Hawthorne, down 9.9 percent; Marshall Heights, down 8.6 percent; Riggs Park, down 7.9 percent, and Takoma Park and Trinidad, each down about 5 percent.

There was a 13.6 percent increase in residential assessments in the city's downtown area and a 9 percent increase in Foggy Bottom, although most of that reflected new construction and condominium development. Other areas recording increases in assessed value included Lily Ponds, Forest Hills, Fort Dupont Park and Wesley Heights.