The growth of retail sales in the Washington metropolitan area slowed dramatically last year to 8 percent from 12.5 percent in 1977, the Commerce Department reported yesterday.

Washington stores sold an estimated $12.2 billion worth of goods in 1978, up from $10.8 billion in 1977, according to the government estimates.

The sales growth rate of Washington area merchants trailed both the national average of 10 percent and the 12 percent gain reported for the South Atlantic States.

The gtowth in retail sales was also less than the increase in consumer prices in the Washington area, which jumped nearly 10 percent last year.

That means consumers spent 8 percent more money, but bought little if any additional goods. And it means merchants rang up 8 percent more money in their cash registers, but sold about the same number of items.

The brightest sign in the Commerce Department's monthly retail report was the tremendous jump in Washington area department store sales from November to December.

December department store sales increased 64 percent over November figures, reflecting a surge in Christmas spending that largely made up for the turgid pace of fall business.

The Commerce Department estimated that sales of the 104 department stores in the Washington area totaled $235 million in December compared with $143 million in November and $230.6 million in December 1977.

For the year, department store sales were up both in the metropolitan area (to $1.48 billion from $1.4 billion) and in downtown Washington (to $94.5 million from $92 million).

The improvement in downtown department store sales figures reversed a two-year trend that had ben marked by a one percent loss of business in 1977 and a 2 percent decline in 1976.

Downtown merchants have been saying for months that sales have been improving now that the Metro subway system is running on Saturdays.

The Commerce Department's retail sales figures released yesterday were for December and for all of 1978. Most retailers opearte on a January-to-January fiscal year to account for after-Christmas promotions, so it will be another month before the federal figures reflect the full retail year.

Nationally, Commerce reported 1978 retail sales totaled $780.6 billion, which was 10 percent more than the $708.3 billion of sales in 1977.

Durable goods dealers reported the best sales gains, with building materials, hardware, garden supply and mobile home dealers running increases of about 14 percent. With gains averaging 8 percent, home furniture stores did not keep pace with other retailers.