As District of Columbia property tax assessment notices are mailed to thousands of homeowners this week, a citizens organization is charging that houses in well-to-do neighborhoods tend to be underassessed and houses in poorer ones tend to be overassessed.

Citizens for Fair Assessment, which has criticized city tax assessments in five studies during the last seven years, said in a report released yesterday that research on fiscal 1986 property taxes shows that the District's Department of Finance and Revenue is using "inadequate and faulty" assessment methods that frequently distort property assessments.

Among other findings, the citizens organization concluded that city tax assessments since 1982 have less accurately reflected the real market value of properties, after being more accurate from 1980 to 1982.

Citizens for Fair Assessment said that the average sales price of single-family houses increased more than 100 percent from 1977 to 1984, whereas the average assessment for properties sold during that period increased 73 percent.

District officials responsible for setting assessments could not be reached for comment, but in the past they have said that the citizens organization does sloppy research.

An assessment is supposed to reflect the property's market value.

To set assessments, city officials, among other things, try to measure the degree to which a property's assessment reflects the price of homes in the surrounding neighborhood. To set assessments for the 1986 tax year -- the property assessments mailed out a year ago -- city officials consulted the prices of properties sold in 1984.

According to the study, using fiscal 1986 assessments, the ratio of assessments citywide to home sale prices citywide was 89.6-to-100, or 89.6 percent. That essentially means that homes on the average were underassessed by a bit more than 10 percent.

In numerous wealthy city neighborhoods, assessments on the average were much more out of line, the study said.

In Massachusetts Avenue Heights -- for years the city area with the most expensive houses, having average assessments in the $500,000 range -- assessments were 71.7 percent of sales prices. Other well-to-do neighborhoods west of Rock Creek Park were allegedly underassessed: The Woodley area had an assessment-to-sales ratio of 77.6 percent, Spring Valley had 80.6 percent, Wesley Heights and Georgetown had 86.3 percent, Chevy Chase had 85 percent, and Cleveland Park had 84.1 percent.

A number of neighborhoods east of the park were comparatively overassessed, the study found: Anacostia had an assessment-to-sales ratio of 97.7 percent, Brentwood had 102.6 percent, Columbia Heights had 103.9 percent, Eckington had 101.5 percent, Petworth had 100.5 percent, Trinidad had 100.3 percent, and Woodbridge had 103.1 percent.

Robert Stiehler, who helped write the report, said that there is "really no excuse" for the kind of assessment variation that he found among neighborhoods.

"It's relatively easy [for assessors] to get rid of that variation among neighborhoods," said Stiehler, a Chevy Chase neighborhood activist and retired government scientist. He added that city assessors had largely eliminated that sort of variation in 1982.

Citizens for Fair Assessment -- founded by members of Common Cause and a variety of neighborhood organizations -- recommended that the city beef up the budget and staff of its assessor's office.

Revenues to the D.C. government from property taxes in fiscal 1986 were estimated to be $437 million, just less than the amount collected in individual income tax revenue, $442 million.