The average daily cost of hospital care in Northern Virginia rose 16.2 percent between Sept. 30, 1980, and Sept. 30, 1981, from $253.32 to $294.40, according to a report by Group Hospitalization Inc., the Blue Cross Plan for the Washington area.

In the District, the average cost of one day in a hospital rose 13.8 percent, from $347.74 to $395.69. In Prince George's and Montgomery counties in Maryland, the average cost per day rose 16 percent, from $242.38 to $281.21.

The Blue Cross report is based on data received from Blue Cross-contracting hospitals in the Washington area. It computes the cost of a day of inpatient care, including not only room and board but also other hospital charges such as medications, use of operating and treatment rooms and other special services.

On a national basis, the American Hospital Association reported that hospital costs throughout the country rose 18.8 percent during the same period.

The average length of a hospital stay per Northern Virginia patient increased during the time period from 7.3 days to 7.5 days, and the average hospital occupancy rate increased from 80.2 percent to 81.3 percent.

In the District, the average hospital stay decreased from 9.5 days to 9.3 days, and the average rate of occupancy decreased from 83.3 percent to 82.5 percent. In Prince George's and Montgomery counties, the average stay decreased from 7.4 to 7.3 days, and average occupancy increased from 74.5 percent to 76.8 percent.