Construction of new homes and apartments, a traditional bellwether of the economy, soared 5.6 per cent in October to register the strongest performance in 4 1/2 years, the Commerce Department said yesterday.

New housing construction last month was started at a seasonally adjusted annual rate of 2.18 million units. That is the most units under construction in any month since May, 1973, when 2.67 million units were started, a government economist said.

The October level was 27 per cent ahead of the same month a year ago and 5.6 per cent above the revised September rate of 2.06 million units, the department said.

Construction of single-family homes set a monthly record with 1.55 million units compared with the September rate of 2.06 million units, the department said.

"October was a great month," said Micahel Sumichrast, chief economist for the National Association of Home Builders.

He said the two main reasons for the continued strong housing performance were that young coupled have flooded the housing market and money is available to finance the purchases.

e warned, however, that "unfortunately, it won't continue." Sumichrast foresees a slowdown in 1978 because the inflow of deposits at savings and loan associations and mutual savings banks has declined from year-earlier levels. That money has been a traditional source of housing money.

In October, the Commerce Department said, apartments buildings with five or more units were started at a 498,000n annual rate compared with September's 416,000 unit performance.

During the first 10 months of this year, 1.7 million units were actually started compared with 1.3 million over the same period in 1976, a 31 per cent jump, the department said.

Building permits were authorized at an annual rate of 1.85 million units last month, 9 per cent above the revised 1.69 million level in September.

The Commerce Department said it takes about three months to establish a trend for housing starts, and noted that housing starts rose 10 per cent in July and were virtually inchanged in August and September. For the past four months, construction has been at an annual rate above million, well above the 1.6 million rate established throughout the 1960s.