For the first time in recent decades, the average price of resold homes in Northern Virginia dropped in March, the Northern Virginia Board of Realtors reports.

Mary Howard, board president, attributed the decline to lack of sales in the high-priced range rather than to a decrease in appreciation of those homes that did sell.

The average for the month was $99,288, showing a 10 percent drop from the $109,825 average recorded a year earlier, the Realtors reported.

But the real decline in home prices actually is understated because of the significant concessions on financing that sellers now offer and because of value lost to inflation, real estate experts say.

Kenneth Kerin, research analyst at the National Association of Realtors, said if seller concessions alone are taken into account the average home price would show another 5 or 7 percent decline in addition to any recorded drop.

Nationwide, recorded sales prices are about the same now as they were a year ago, but measurable declines have started showing up in places hit harder by the recession and unemployment, Kerin said.

"The Washington suburbs no longer have a recession-proof economy. Unemployment is a problem, and we are seeing the same kinds of effects on home prices as in Chicago, Milwaukee and Detroit," Kerin said.

But Howard said that to a certain extent Northern Virginia's price figures are misleading because of how they are compiled.

"The high-priced homes haven't been selling," and this leads to a distortion in the statistics, Howard said.

As a result, the way the board keeps and reports its figures will be changed next month. From now on, the price figures will be broken down into prices for condominiums, for town houses and for single-family homes, Howard said.

Taking into account inflation and seller-financing concessions, real home prices have been dropping for some time. But these factors do not show up in reported sales price figures, so these figures generally have continued to record a modest increase.

One exception was in Montgomery County, where a measureable decline was recorded for the first time last year, according to real estate consultant Alfred W. Jarchow, publisher of the Trends in Home Prices report out of Rockville.

There prices reached a peak last May and then declined 2.6 percent from May to December, Jarchow said. The net result was a recorded price rise of a miniscule 1.4 percent from December 1980 to December 1981.

The Prince George's County Board of Realtors reported that average prices of single-family homes were one percent higher in March than a year earlier and that condo prices were 9 percent higher.

March sales in Prince George's County were 19 percent lower than the year before, with single-family homes sales declining more than condo sales.

Sales in Northern Virginia in March were 37 percent lower than a year earlier, and year-to-date figures were 39 percent lower than in 1981.

The price and sales figures also do not reflect "kickouts," or sales that do not go to settlement because the buyer cannot qualify for a mortgage.

Meanwhile, Housing Data Reports released figures showing that the prices of new homes in the Washington area recorded and overall 1.4 percent rise from March 1981, to March 1982.

In the past quarter, however, average new-home prices dropped 13 percent from $118,800 in January to $117,300 in March for the Washington area as a whole.

The figures show price declines at new homes in three jurisdictions, the District, Alexandria/Arlington and Montgomery County. The District average took a drop of 16.4 percent, from $247,500 to $206,900, but Housing Data Reports said this was not representative because the sample was small.

The largest increase in the area was in Prince William County where average new-homeprices increased 7.2 percent in the past year.

HDR attributed the price drops to sales being concentrated in smaller, cheaper homes and to some large price reductions by only a few builders. The cuts were "highly noticeable" ut occurred at only 18 projects of 550 surveyed, HDR said.