Experts in the private real estate industry said yesterday that the 13.5 percent average increase in D.C. residential real estate assessments reflects a market that is experiencing brisk sales of homes.

"There has been an increase in what people are willing to pay for property," said David Strachan, executive vice president of the Washington Association of Realtors. "D.C. bases its tax assessments on comparable sales. In a market like Washington is experiencing right now -- one driven by relative low {mortgage} rates and consequent high prices -- what you find are people willing to pay a lot . . . . It's definitely a seller's market."

Rufus S. Lusk III, who heads a real estate information service, said areas west of Rock Creek Park have had the most significant price jumps -- a trend that is reflected in the higher assessments in those areas. Overall, Lusk's service reports that the value of homes sold in 1987 was 13.6 percent higher than in the previous year.

The new assessments, more than double the 5.5 percent increase of the previous year, have drawn angry complaints from city property owners and suggestions from some D.C. Council members that a reduction in the property tax rate may be in the offing.

"I have never appealed it before, but I intend to {appeal} this time," said Elizabeth Seiler, 64, whose Northwest home was assessed 29 percent higher.

Residents can appeal their assessments to city tax assessors or to the D.C. Board of Equalization and Review, an independent agency.

City officials, commenting on the new assessment figures that were released Wednesday evening, said that varying market conditions in the city's 56 neighborhoods account for sharp variations in the increases. Some homeowners have seen no rise in their assessments, while others reported increases as high as 39 percent.

Charles Horwitz, acting chief of the city's assessment division, said yesterday that the residential assessments, which were mailed last week to property owners, are computed by city assessors based on the sales of about 6,000 homes during the year that ended July 1.

Horwitz said city assessors did a study of the sales in all 56 neighborhoods. Assessors visited some sites, contacted buyers when possible and gathered other pertinent information about the sale to determine the amount of the assessment, he said.

Tax breaks are available for some owners of District residential property. Under the homestead exemption program, eligible homeowners don't have to pay tax on $15,000 of the assessed valuation.

Senior citizens, 65 years and over, also are eligible for a 50 percent cut in their property tax if they live in the house, own at least 50 percent of the property and depend on retirement and Social Security income for at least half of their support.