Residential property tax assessments in Washington have increased an average of 9.4 percent--less than half the increase reported last year--as high interest rates and a slumping economy slowed sales and held prices down, city officials said yesterday.

Still, the figures announced yesterday mean that the average tax bill on a single-family home will exceed $1,000 for the first time, according to officials. They said the average house in Washington is now valued at $91,095, compared to the average of $82,789 a year ago, and the average tax bill will rise from $900.34 to $1,001.56.

The relatively small increase in assessments reflected a dampening of the city's booming housing market of late 1980 and the first nine months of 1981, the period on which the assessment figures released yesterday were based. Moreover, city officials said they forsee no quick recovery and projected an even smaller increase next year.

Commercial property tax assessments rose by 23.5 percent, however, only a slight decline from a 25.1 percent increase reported last year. Officials took that as an indication that this sector is holding its own.

The 9.4 percent average residential increase, which will be reflected in tax assessment notices that the city mailed out yesterday and in tax bills property owners will receive late in the summer, fell sharply from the 25.4 percent averge increase recorded the previous year.

It was also far below the average 21 percent annual increase in value reported for District homes when the years since 1977 are considered together.

"I'm not surprised," James G. Banks, executive vice president of the Washington Board of Realtors, said of the assessment data. "It reflects a slowdown in real estate sales and prices."

Banks added, "There is evidence that there are lots of people who want to sell and a lot who want to buy" homes in Washington. But he said many would-be buyers are "frightened about the future" because of mortgage rates and the sluggish economy.

Some neighborhoods showed increases far above the average, including LeDroit Park, 24.1 percent; Forest Hills and Eckington, both about 20 percent; and Trinidad, Michigan Park and Massachusetts Avenue Heights, all in the 17 percent range. Neighborhoods below the average were American University Park, 3.1 percent; Chevy Chase, 6 percent; Capitol Hill, 5.5 percent; and Petworth and Burleith, both about 4 percent.

Carolyn Smith, director of the city's Department of Finance and Revenue, said city assessors expect the residential market to remain soft into next year and predicted that assessment increases then may be as low as 4 percent.

But Smith pointed to the commercial assessments as proof of a strong overall local economy, noting that "for several years in a row, prior to last year, the highest average was 10 percent."

Assessments on residential and commercial property are based primarily on sales and income figures reported in calendar year 1980 and the first nine months of 1981, according to city officials.

A group of local organizations, including Common Cause, has planned a press conference Saturday to challenge the assessments. The coalition, Citizens for Fair Assessments, contends that two of three assessments in the District are inaccurate and that officials unfairly discourage residents who want to appeal their assessments.

Luis Zapata, a spokesman for the group, said few citizens know that tax assessments primarily are based on values reported more than a year ago. He said more citizens do not challenge assessments because they incorrectly compare the assessed value to the current market values of their homes, which are usually higher. "They think they are getting a tax break," Zapata said. "They are not."

Zapata's group is expected to release results of a study contending that lower-priced homes tend to be overassessed while condominiums tend to be underassessed.

The largest increase in assessed value in residential areas was reported in the community of LeDroit Park, where average values rose from $44,139 to $54,810, a 24.1 percent increase that means an average tax bill about $178 higher. The tax rate on owner-occupied residential property in the District is $1.22 per $100 of assessed value.

LeDroit Park is populated largely by older, middle-income black families, but has seen a sharp influx in the number of younger, more affluent blacks and whites.

The neighborhood with the lowest increase was American University Park in upper Northwest. The average home there rose in value about 3.1 percent, up from an average of $138,774 to an average of $143,100.

Smith said many of the city's more expensive neighborhoods reported smaller increases in assessments because they have "gone through a cycle" of rising values that less affluent neighborhoods are only just beginning.

The largest increase in commercial property was recorded in the central business area, centered on K Street NW, where assessments rose 44 percent, reflecting a continuing growth in the number of new office buildings.

John T. O'Neill, executive director of the Apartment and Office Building Association, criticized the new assessments.

"We already see a shift of major construction" from the District to Northern Virginia, he said, contending that "50 to 55 percent of new construction in the area is taking place in the Virginia suburbs compared to 35 percent in the District.

He said land, taxes and permit fees are lower in Virginia than in the District. The new assessments, which he called "a small break" from the previous year, would not have an immediate effect on the city's business climate.

"Just less and less development will occur over the years," O'Neill said. "Not that many tenants have to be downtown anymore."