Retail sales in the Washington area during November were at the lowest level since last April -- but the area's department stores had their busiest month of the year.

According to the Commerce Department, sales at all types of retail outlets here -- food stores, car dealers, restaurants, general merchandise -- totaled $1.038 billion in November, down 1 percent from October and the lowest monthly volume since $964 million of sales last April.

However, the region's 104 department stores posted a monthly volume of $144 million, a gain of about 12 percent from sales in September and October.

Despite the overall slowdown in sales as consumers obviously concentrated their spending on holiday gifts during November, areawide retail sales that month were up 12 percent from the same month a year earlier. For the first 11 months of 1978, area sales totaled about $11 billion -- a jump of more than 14 percent over the same months in 1977.

Nationwide, retail sales rose in November by 12 percent from a year earlier and for the first 11 months, U.S. retail sales were up 10 percent, indicating that local retail sales increases early in the year account for a significantly higher rate of increase here.

Downtown Washington department stores participated in the general sales rise, in contrast with previous months, posting November sales of $8.8 million -- up 7 percent from a year ago and up 3 percent from October. It was the best month of 1978 for the three major downtown stores -- Woodward/Lothrop, Hecht's and Garfinckels.

In contrast, downtown Baltimore department store sales plummeted 10 percent from the previous year to $3.6 million. Stewart & Co. announced this month that its downtown Baltimore store is being closed, and another major store, Hochschild Kohn, closed a year earlier. That leaves only two major downtown Baltimore department store operations -- Hutzler Brothers and Hecht's.