New home construction plunged 11 percent last month, the largest decline in nearly a year, but the drop reflected a falloff only in apartment building, the Commerce Department reported yesterday.

Construction of single-family homes rose 5 percent last month, while the rate for buildings with two to four units dipped about 12 percent. The rate of construction for large multifamily buildings -- those with five or more units -- dropped 38 percent, according to Commerce Department data.

At the same time, the Federal Reserve Board said that consumer installment credit jumped $7.22 billion in January -- an annual rate of increase of nearly 20 percent and a return to the levels of most of 1984.

The increase was the third-highest ever, trailing only the record $9.09 billion rise of last May and the $8.83 billion jump of last March.

The continued willingness to take on debt indicated that Americans remained optimistic about the future of the economy, according to analysts.

And despite their slide, the housing starts numbers were not regarded as particularly bad news by industry officials.

"We don't read too much into this decline. It's all in multifamily, and the numbers were really inflated in January," said a National Association of Home Builders spokesman.

Warren Lasko, executive vice president of the Mortgage Bankers Association of America, termed the overall 11 percent decline "misleading," noting that the annual rate of housing starts has been running 1.6 million to 1.7 million most of the past year, but then jumped to 1.8 million last month.

"The 1.638 million for February is almost identical to December [and] little changed from August," he said.

However, Lasko and others said they are worried about the current upward drift in interest rates.

The average current rate for 30-year fixed-rate loans, which had been down close to 12 3/4 percent last month, was back to 13.2 as of last Friday, according to a survey by the Federal Home Loan Mortgage Corp.

"The upward drift in interest rates is going to affect those single-family starts," Lasko said. "It's getting to the point where we have affordability problems again."

He said that, with the combination of huge federal borrowings, disturbing news on the financial scene and the continuing robust performance of the economy, he sees no reason to expect any softening of rates.

The Commerce Department reported that building permits, usually a precursor of future construction, declined 5 percent in February to an annual rate of 1.6 million.

A total of 200,000 housing units were started in the first two months of the year, a decrease of 16 percent from the same period in 1984.

The NAHB said its March surveys of builder members also show much less optimism about market conditions than those a month earlier.

Although Commerce Secretary Malcolm Baldrige said yesterday that he expects housing starts to reach 1.8 million this year, up from 1.75 million last year, Gopal Ahluwalia of the NAHB's economics department said he thinks "we'll be lucky to reach" the association's current forecast of 1.65 million.

"We expect interest rates to start going up in the summer, probably to 14-percent-plus," he said.

The result will be that 1985 will be "a mirror of last year," with a vigorous housing industry in the first half and much less activity in the second, he added.

The Federal Reserve's consumer credit report shows that Americans' installment debt now totals $459.24 billion, an increase of 20.5 percent from a year ago.

Auto loans in January increased by $2.89 billion in January, compared to with $2.69 billion in December, while short- and medium-term personal debt such as bank loans rose $2.54 billion, down slightly from December's $2.57 billion in December.