Washington retained its title as the nation's most affluent marketplace in 1979, according to the arbiter of such data, Sales & Marketing Management Magazine.

In the publication's annual survey of "buying power" throughout the nation, now being distributed, the average aftertax income of households in metropolitan Washington is listed at $27,200. That's the highest in the ten largest urban markets and 31 1/2 percent above the U.S. average.

Local retailers will find little to cheer about in the new report, however. True, total after-tax spendable income here rose 10 1/2 percent last year to $29.5 billion, ranking Washington the eighth largest market.

But overall retailer sales in 1979 increased only 8.9 percent over 1978 to $13.6 billion, a gain of $1.1 billion. Consumer prices rose at about the same rate. Add up all of the figures and they indicate that area consumers are devoting a smaller share of their spendable income to retail purchases, including food.

Soaring transportation, energy and housing costs effectively are claming a bigger portion of the average household income left to use after taxes are deducted.

Over a longer term, the full decade of the 1970s, income after taxes here increased $17 billion and average household income more than doubled the 1970 figure of $13,539 as the cost of living rose by a somewhat slower rate of 85.4 percent.

According to Sales & Marketing statistics, which may have to be modified a bit when complete U.S. census figures are available, the number of area households increased by 161,600 or 19 percent since the end of the last census year, 1970, to an estimated 1.086 million at the end of 1979.

The three Maryland countries in the official D.C. statistical area (Montgomery, Prince George's and Charles) had household gains of 76,600 or about half the overall increase. Fairfax County led all local communities with an increase of 47,000 households.

In the key measure of jurisdictional affluence, average household incomes, Fairfax continued to beat out Montgomery by $33,578 to $30,333.

Jane Huffman, of The Washington Post's research staff, observed that in terms of both household and income growth here, Maryland and Virginia portions of the metropolitan area have been fairly even. But within these state groupings, the largest relative gains have been made in the three more rural countries (see adjacent chart).

These three jurisdictions, Prince William and Loudoun in Northern Virginia plus Charles in Maryland, also are the most recent additions to the standard statistical area and farthest from the central core.

Among other details from the annual Sales & Marketing study, as compiled by Huffman:

Per capita income in 1979 here was $9,667, highest among the top ten urban markets and 32 percent above the U.S. average.

Per household retail sales were $12,562 in the area last year, or about 11 percent above the national average.

Per household food store sales averaged $2,493, the third-highest amount in the major markets. Supermarkets were the biggest single recipient of consumer spending in 1979. A close second here was auto dealers and supplies (including gasoline stations), at $2,393 per household; third is general merchandise retailers, at $1,800 a household.

Maryland counties in the metropolitan area account for 32 percent of the state's population, 31 percent of households and 36 percent of all retail sales. Virginia jurisdictions in the area account for 20 percent of that state's population as well as households by 25 percent of retail sales.

Among the 10 largest urban markets, the closes to Washington in spendable income per household are Houston, $25,591; Detroit, $25,685; and Chicago, $25,155. New York, the largest market with 3.6 million households, is tenth in per-household buying income at $20,700. CAPTION:

Illustration 1, no caption, by Dan Sherbo -- The Washington Post; Chart, HOUSEHOLDS AND INCOME WASHINGTON METROPOLITAN AREA, by Glenn Mosser -- The Washington Post; Illustration 2, no caption, By Dan Sherbo -- The Washington Post