WASHINGTON (AP) _ Sales of previously owned homes fell 6.4 percent in January, after a December advance that had boosted sales to their highest level in nearly 20 years, a real estate trade group said Monday.

The drop, the first since a 1.2 percent decline last August, was felt in every region of the country. But another report Monday showed that spending on residential construction rose in January for the sixth straight month.

A survey by the National Association of Realtors showed that sales of existing single-family homes totaled 3.78 million at a seasonally adjusted annual rate, down from 4.04 million in December.

The December rate, even higher than the 4.02 million original estimate for the month, had pushed sales to their highest level since they reached 4.09 million in May 1979.

''The number of 4 million was not a sustainable number,'' said Daryl Delano of Cahners Economics in Newton, Mass. ''It overstated the strength of the market.''

But Delano and Realtors economist John A. Tuccillo agreed that housing market fundamentals will support moderate growth this year.

''The stage is set'' for a healthy year in the housing industry, Tuccillo said. ''All the factors are in place - low rates, stable home prices and willing consumers.''

Fixed-rate, 30-year mortgages averaged 8 percent in January, down from 8.21 percent a month earlier, according to surveys by the Federal Home Loan Mortgage Corp. Rates had fallen to 7.53 percent by last week, the lowest since they averaged 7.51 percent during the week ended April 6, 1973.

The Realtors said the median price of a previously owned home in January was $103,700, down 0.5 percent from December, but 1.6 percent above that of a year earlier. The median means half of the homes cost more and half cost less.

Sales of new homes jumped 6.3 percent in the final month of 1992. The government will announce the January figures Tuesday.

In the other report Monday, the Commerce Department said construction spending fell 1.3 percent in January, to a seasonally adjusted $430.8 billion annual rate. It was the first decline in five months.

Spending rose 1.1 percent for residential construction. Outlays for single- family homes increased 1.4 percent, but spending on apartments plunged 11.9 percent.

Spending on non-residential and government buildings also fell, down 1.9 percent and 4.9 percent respectively.

Analysts said the drops in apartment and non-residential spending reflected the overbuilding in the late 1980s and early 1990s.

Despite the drop, the Realtors said existing home sales in January were 14.9 percent higher than those of a year ago. They are projecting 3.69 million sales this year, the most since 3.83 million in 1979. Sales totaled 3.52 million in 1992, also the highest since 1979.

Regionally, sales fell 8.7 percent in the South, to a 1.36 million annual rate. They were down 6.9 percent in the West, to 810,000, and 4.8 percent in the Northeast, to 600,000. Sales declined 3.8 percent in the Midwest, to a 1.01 million rate.

But the median price of a previously owned home rose 1.2 percent in the Midwest, to $82,600. However, it dropped elsewhere - down 2.4 percent in the South, to $90,400; 1 percent in the Northeast, to $134,400, and 0.3 percent in the West, to $143,300.