Despite a surge in department store sales during December, total Washington area retail volumn last year rose only a modest 8 per cent over 1975 sales, according to initial estimates by the Commerce Department.

For the year, metropolitan area sales at all establishments - food stores, general merchandisers, auto dealerships and restaurants - totaled $9.56 billion compared with $8.83 billion in 1975. Consumer prices rose 4.8 per cent last year, indicating that Washington area sales of actual merchandise rose only slightly more than 3 per cent during 1976.

In the month of December alone, areawide sales totaled $1 billion compared with $915 million the previous year, an increase of 10 per cent.

In December 1975, sales were up 13 per cent from the 1974 period and for all of 1975, areawide sales rose 9 per cent over the previous year. Nationwide, during 1976, total retail sales were up 12 per cent - reflecting a general malaise in the D.C. area in contrast to greater sales growth elsewhere during an election year.

At 92 area department stores surveyed regularly by the Census Bureau, sales growth for all of 1976 matched the general pattern, with an increase of 8 per cent to $1.3 billion. In December, however, department stores here were up 14 per cent from a year ago to $201 million.

For the three major downtown department stores, 1976 was not a good year. Volume for the year feel by 2 per cent to $93.5 million at the downtown units of Woodward & Lothrop, the Hecht Co. and Garfinckel's, although sales in December alone were up 1 per cent to $13.6 million.

Of nine central city districts surveyed monthly by the Commerce Department for downtown department store sales, Baltimore suffered the greatest decline in 1976 - off 9 per cent to $53 million. Sales declined by 7 per cent in downtown Boston and by 1 per cent or 2 per cent in downtown Chicago, Cleveland, Los Angeles and Pittsburgh. In contrast, downtown sales rose 2 per cent in Oakland and 4 per cent in Philadelphia, the government reported.

Retail volume in 1976 also was weak in metropolitan New York and north-eastern New Jersey, with December sales up only 4 per cent and total annual sales up 5 per cent - just matching the rise in the retail prices. CAPTION: Picture, ARTHUR F. BURNS . . . $10 billion more