Housing starts dropped by 13 percent in December from the previous month to a seasonally adjusted annual rate of 1.22 million, but the decrease was not enough to erase the past seven months' gains, the Commerce Department reported yesterday.

Housing industry spokesmen said the December figures therefore are good news and predicted that improvement will continue. The annual rate of construction starts now has topped the 1 million mark for seven of the last eight months.

The growth did not come in time to prevent 1982 from being the worst year for the industry since 1946, however. The total number of starts last year was 1.061 million, only slightly higher than the 1.015 million reported in 1946.

Last year's starts were 2.2 percent below those of 1981, and the record of 1982 "would have been even worse if we hadn't had an upturn in the whole second half of the year because of lower mortgage interest rates," said Mark J. Riedy, executive vice president of the Mortgage Bankers Association.

Riedy characterized November's rise of 24.7 percent in construction starts as "a blip." He said the sharp rise in November can be attributed in part to "the tail end of starts on some government-assisted multifamily programs."

Construction starts of single-family homes dropped 9.4 percent to an annual rate of 800,000 last month compared with a rise of 26 percent in November.

Housing permits rose 8.3 percent in December to an annual rate of 1.29 million, the Commerce Department said. Riedy saw the increase as a sign of housing health and said the industry "is the only really bright spot" in the economy that should be nurtured by the administration and Congress.

The head of the National Association of Home Builders, Fred Napolitano, said builders are "cautiously optimistic" that "the trend over a four- or five-month period, which shows a substantial increase in housing activity," will continue.