Homebuilders and real estate agents finally got some long-awaited good news this week in some upbeat national forecasts for interest rates and home sales for 1982.

Sales of existing homes will jump 20 percent to 2.9 million units in 1982, with mortgage rates falling below 14 percent, a forecast by Multi-List McGraw Hill said. Average home prices are likely to rise 7 percent to $84,100, according to the reporting service, a division of McGraw-Hill Information Systems Co.

The report says there is a "huge, fundamental demand for homeownership among first-time buyers" that will boost sales next year.

However, Washington-area figures released this week showed the market here still in decline.

The Northern Virginia Board of Realtors reported that sales of single-family homes in that area in October were 48 percent lower than a year earlier. For the month of October, there were 834 sales this year, compared with 1,606 last year, the group reported.

The group's analysis of the figures is a study in looking on the bright side. The 48 percent drop "was not as severe as might have been expected. . . . We can be glad it was no worse." It also said that "despite dire predictions," sales by Northern Virginia Realtors through October were down "only" 20 percent from last year's levels.

Average prices rose 5 percent from October last year to October this year, to $94,953.

In Montgomery County, house resale prices increased by an average of 4 percent over the past 12 months--compared with annual rates that were as high as 16 percent in 1979-- but took their first actual decline--1.2 percent--in nominal terms in the third quarter of this year, according to local real estate consultant Alfred W. Jarchow.

In addition, a convention of the U.S. League of Savings Associations in New York heard cheery predictions from its president, a top Reagan administration official and the head of the National Association of Home Builders.

Murray Weidenbaum, chairman of the President's Council of Economic Advisors, assured the group that interest rates will fall through the middle of next year, giving a boost to the housing industries. He credited the Federal Reserve Board's tight monetary policy for recent declines in rates.

Savings league President Rollin D. Barnard and homebuilders' President Herman Smith predicted that interest rates would fall to about 14 percent, which they said would help first-time home buyers enter the market.

Barnard told the group it would be a while, however, before the influx of money to S&Ls from All Savers certificates would result in lower mortgage rates. While the cost of money started to come down at the end of October, it will have to decline further before S&Ls will be making profits again and can reduce their rates on mortgages, he said.

Smith said a 3 or 4 percentage point drop in interest rates would set the stage for a comeback of the housing industry, including housing starts of about 1.5 million units next year.