Residents of the rural counties of northern Virginia and southern Maryland are getting richer faster on the average than others in the Washington area, according to Sales and Marketing Management magazine.

In its annual "buying power" survey, the magazine reported that average household incomes are increasing most rapidly in Maryland's relatively countified Charles County and Viriginia's Prince William and Loudoun counties.

The Washington area retained its No. 1 ranking in per capita income among the nation's ten most populated urban centers in 1976, the survey showed. Per capita income here averaged $7,316 last yea, or 35 per cent above the U.S. average.

In other comparisons among the major markets. Washington emerged as one of the most popular places to live.It ranked second behind the Dallas-Fort Worth area in population growth since 1970, and third in household growth.

In the average household income rankings, the Washington area held onto second place behind New York's Nassau and Suffolk counties, which comprise the bulk of Long Island, Nassau-Suffolk household income was $22,501 last year compared with $21,149 in metropolitan Washington. That's nealy 33 per cent above what the U.S. average was for the year.

Fairfax County continues to be the richest county in the area, enjoying a comfortable average income per household of $25,792. Montgomerty County took second place with $24,443 per household last year.

But over the last decade, the biggest income gains have been made in the three rural counties of Loudoun (average household income 101.4 per cent to $17,881). Prince William (up 159 per cent to $24,553) and Charles (up 101 per cent to $19,142), according to the survey.

The figure for Prince William has been criticized as too high by county planners, who claim the survey under estimated household growth in the area. "We'd like to believe the incomes here are over $24,000," said James Gracie, a Prince William County planner, "but we thinks it's more like $17,000."

In itw review of 1976 retail business across the nation. Sales Management estimated total Washington sales at more than $10.3 billion, up 7.5 per cent over 1975. That represent retail sales per household of $9,812, which is 9.7 per cent above the national average.

According to the survey, total population for the Washington metropolitan area topped 3,056,000 last year, an increase of 3.9 per cent since the end of the census year 1970. The Maryland suburbs of Washington continue to dominate the area in both the number of housholds (41 per cent of the area's total) and effective buying income (42 per cent).The Virginia suburbs account for 32 per cent of the households and 36 per cent of the buying income in the Washington area.