The average price for resale homes in the Washington area will top $100,000 this year, and average monthly payments on those homes will leap by 19 1/2 percent from last year, the National Association of Realtors predicts.

The $100,900 average for this area is up 10 percent from the 1980 average price of $91,700, but the monthly payments on resale units are rising more dramatically because they include higher interest rates, property taxes, utility costs, and hazard insurance. The projected price average for all of 1981 was based on actual sales in the first six months.

The figures do not take into account the cost to home sellers of concessions in financing terms which increasingly are being offered to attract buyers during the current housing slump. Some analysts believe these concessions amount to price reductions which are not reflected in what they say are inflated home price statistics.

The Realtors figures ranked the Washington metropolitan area third-highest in average existing home prices among 15 cities surveyed across the country, behind San Francisco at $133,900 and Los Angeles at $120,000.

For newly built houses, not included in the Realtors survey, the average price in the Washington area surpassed the $100,000 mark last year, according to Housing Data Reports, a Bethesda research firm. By the end of 1980, the average price of new homes in major subdivisions in the area was $113,700, pushed by a 42 percent surge in prices in the District and more moderate increases in the Virginia and Maryland suburbs, the researchers said.

By May this year, the figure was up to $116,900, but this represented a slower yearly rise for 1981 than in 1980, Housing Data Reports research analyst Anna Czupryn said. One trend the latest figures show is a narrowing of the gap between the prices of new homes close to the District and those in the outlying areas, Czupryn noted. The research firm tracks only prices in major new subdivisions and does not include condominiums.

Nationally, average new-home prices hit $88,300 in June, up by $4,000 from May, the Commerce Department reported earlier. The Realtors association estimated that nationally, the average price on existing homes in May was $78,600.

The unabated climb in housing prices has continued despite the current record slump in the housing market, brought on largely because of stunningly high interest rates.

Existing homes are selling at an annual rate of about 2.6 million units in 1981, down from the 2.9 million units in 1980, the Realtors said.

The Realtors' latest survey compared average prices of homes which have been sold in the 15 selected metropolitan areas. The group based its estimates on figures of various kinds from the Federal Home Loan Bank Board, the Bureau of the Census, the Bureau of Labor Statistics and the U.S. League of Savings Associations as well as the NAR's own statistics.

Monthly payments of principal and interest, at prevailing rates, would be $1,048 on the average price of $100,900 estimated for the Washington area this year, the survey showed. Adding on property taxes, utilities and hazard insurance, the average monthly housing cost here is estimated at $1,310. That is a 72 percent change from the area's average monthly cost of resale property in 1978, which was $761.

The Washington area fared better in relation to other cities when average home costs were compared with average income. The total average annual home cost here is expected to equal 39.6 percent of gross family income in 1981. This was up from 31.4 percent in 1978. By that measure, Washington ranked No. 7 on the list of 15, right behind Baltimore.

While Baltimore's average price for resale homes in 1981 is estimated at $72,100, 10th-highest out of 15, average income in that metropolitan area is $28,440. Therefore, the total housing costs as a percent of income would be 40 percent in 1981, higher than Washington's.

"The deterioration of housing affordability over the last three years has been devastating," the Realtors' chief economist, Jack Carlson, said.

The survey showed that since 1978, housing prices in the 15 metropolitan areas surveyed rose between 62 percent (in Detroit) to 106 percent (in San Francisco).

The lowest average resale price of the 15 cities in 1981 was in Pittsburgh, at an estimated $59,000 or $601 a month principal and interest at prevailing rates. When property taxes, utilities and hazard insurance were thrown in, however, St. Louis ranked lower in average monthly housing costs at $815.

Average prices and effective mortgage rates for the survey were based on data from the Federal Home Loan Bank Board, and principal and interest payments were based on an 80 percent, 30-year loan.

The rankings of other cities for average estimated 1981 home prices, starting with the highest, were: San Francisco, Los Angeles, Washington, Houston, New York, Minneapolis, Milwaukee, Chicago, Atlanta, Baltimore, Boston, Philadelphia, Detroit, St. Louis and Pittsburgh.