HONOLULU -- The median cost of a resale home in the Washington area escalated 9.6 percent to $108,400 in the year ending Sept. 30, one of the highest rates of appreciation in the country, a new National Association of Realtors survey shows.

Despite the increase over the past year, the median price of a Washington area house -- half cost more and half cost less -- dipped 5.7 percent from $115,000 in the April-to-June period compared with the July-to-September period.

The midyear drop was apparently a more pronounced reflection of a nationwide trend, in which the Realtors said resale home prices generally leveled off in the wake of higher interest rates last spring. Interest rates have dropped again in recent weeks after the stock market's huge plunge on Black Monday, Oct. 19.

Sales activity was high during the second quarter in the Washington area, which boosted prices, said Glenn Crellin, the Realtor group's vice president for economics and research. He discounted the significance of the drop of the median price from the second quarter to the third, saying, "The key is the yearlong number. Nearly 10 percent {increase} is a very high rate."

Higher mortgage rates also caused sales of existing homes to drop across the nation in the third quarter, the trade group said. The seasonally adjusted annual rate of resales was 3.8 million for the July-to-September period, down 3.3 percent from the third quarter last year and off 4.8 percent from the April-to-June period this year.

"Some would-be buyers found themselves priced out of the market," said Realtors President William M. Moore. "For them, the smallest mortgage rate jump often means the difference between owning and renting. Sales generally slowed down when rates failed to fall last summer by as much as they went up last spring."

The Washington area's $108,400 median price for a resale detached home or town house makes it the ninth most costly housing market in the 53-city survey. The New York area tops the list at $183,000, followed by Boston at $181,600, San Francisco at $175,900, Orange County, Calif., at $167,700, Hartford, Conn., at $165,400, Los Angeles at $139,600, San Diego at $131,700, and Providence, R.I. at $126,600.

The cheapest housing listed in the survey was in Louisville, $53,300, followed by Grand Rapids, Mich., at $54,000 and the Buffalo-Niagara Falls area in New York at $55,900.

The national median was $85,400, up 5.8 percent over the $80,700 median in the third quarter of 1986. Thirty of the 53 metropolitan areas surveyed showed rates of appreciation higher than the national average, according to the Realtors.

Providence's appreciation rate was the highest at 37.8 percent, reflecting an increase in the median price there from $91,900 in the third quarter of 1986 to $126,600 in the same period this year.

Hartford housing prices jumped 25.3 percent during the year, the Albany-Schenectady-Troy, N.Y.-area prices rose 19.1 percent. Prices in Las Vegas fell the most -- 5.7 percent -- with Toledo second worst with a 5.5. percent drop.

John A. Tuccillo, the Realtors group's chief economist, said, however, that the pockets of high appreciation are camouflaging an overall price stall. He said that while the Northeast will continue to have the highest concentration of high housing prices, there will be slower appreciation.