The number of blacks living in suburbs increased more in the Washington area during the 1970s than in any other metropolitan area in the country, with families that include school-age children accounting for most of the growth, according to new analyses of the 1980 census.

The total of black suburbanites here reached 404,814 in 1980, up 224,405 in a decade, the reports said, to give the Washington area the largest number of suburban blacks in the nation.

The metropolitan area ranks eighth in the nation in total population and sixth in the total number of blacks.

"From what we see the increase was mostly younger black families who want better schools and better opportunities for their children--the same as everybody else does," said John McClain, director of community and economic resources for the Metropolitan Washington Council of Governments, which issued one of the reports.

According to the new COG data, the number of black children in the suburbs from ages 5 through 17 rose by 53,830 over the decade, compared to a drop of 49,016 in the number of black children in this age group in the District.

There also was a substantial increase in black preschool children in the suburbs at the same time as they fell sharply in the District. Working-age black adults (ages 18 to 64) almost tripled in the suburbs while their numbers fell 6 percent in the city.

Elderly blacks over age 65 were the only group to increase in both the city and the suburbs.

William O'Hare, a demographer who used census data to prepare a nationwide survey of black suburban growth for the Joint Center for Political Studies, said the rapid growth here probably comes from a combination of relatively high incomes and the "natural expansion of black neighborhoods" across the District line.

According to data compiled by O'Hare, the increase in blacks in the Washington suburbs was almost 50,000 more than the 157,822 growth around Los Angeles, which had the second-largest gain in black suburbanites over the decade.

The Atlanta suburbs were third with an increase of 123,352 blacks, followed by Chicago with a gain of 102,528.

O'Hare said much of the black suburban growth nationwide is "simply an extension of central-city neighborhoods into new political juridictions," and added, "There's a real question about how much sociological change is going on."

O'Hare pointed out that about two-thirds of the black increase in the Washington suburbs was in Prince George's county--which, at 37 percent, had the highest proportion of blacks of any major suburban county in the nation.

The data compiled by COG showed sharp demographic differences between District and suburban blacks. The number of black married couples with children under 18 in the suburbs more than doubled during the decade to 42,065, for example, while it dropped by almost half in the city to 23,468.

Also, as the suburban black population soared, the proportion below the poverty line fell from 14.5 percent in 1970 to 10.3 percent in 1980. In the District, where the total black population fell by 16.5 percent, the proportion of poverty-level blacks in the city rose over the decade from 19.1 to 21.4 percent.

In Prince George's, where the black population rose the most, the proportion of blacks below the poverty line fell from 10.1 to 8.4 percent.

Another report, prepared for the Greater Washington Research Center by demographers George and Eunice Grier, shows that areas outside the Beltway accounted for 39 percent of the total black suburban increase during the 1970s.

"A great deal of the movement of blacks into the suburbs here was into new housing," George Grier said in an interview. "Yes, there are older areas inside the Beltway that are becoming pretty heavily black. But there are a lot of areas to the outside where both blacks and whites are moving in."