Construction of new homes jumped 8.4 percent last month, reaching its highest level in nearly five years, the Commerce Department said yesterday.

The large rise in housing activity reflected lower interest rates and increased consumer confidence during the late spring and early summer when many homebuilding plans were made, economists said.

However, the economists also were quick to note that the one-month gain for the usually beleaguered industry may be short-lived, because higher mortgage interest rates are sure to blunt further increases in new home construction.

Privately owned housing units were started at an annual rate of 1.935 million, up from the revised July figure of 1.785 million. It was the highest level of activity since December 1978, when new homes were started at an annual rate of 2.044 million.

"The reports of the demise of this industry are greatly exaggerated," said Commerce Department Chief Economist Robert Ortner.

But he added, "there will be very little, if any, further growth" with mortgage interest rates at about 14 percent.

"Mortgage interest rates in the 14 percent range are putting housing credit beyond the reach of a growing number of potential home buyers," said Jack Carlson, chief economist of the National Association of Realtors. "Only when the burgeoning federal budget deficits are put on a downward path toward balance will mortgage interest rates fall to more affordable levels."

In addition to rising mortgage interest rates, another indicator of future depressed housing activity was a 6 1/2 percent drop last month in the issuance of building permits, which usually is reflected several months later in the start of new home construction. Sales of new homes also have declined within the last few months.

Building permit authorizations for single-family homes grew at a seasonally adjusted rate of 913,000 last month, virtually unchanged from July, the Commerce Department said. A 14 percent drop in the rate of authorization for buildings with five units or more was blamed for most of the decline.

"Starts by the end of the year are going to come down," said Harry Pryde, president of the National Association of Home Builders.

The August level of housing starts was "a rather robust recovery" compared with the 840,000 annual rate recorded in November 1981, several months after the most recent recession began, Ortner said. Instead, growth in the economy will shift away from homebuilding to sectors less sensitive to interest rates, such as consumer spending, Ortner said.

Starts of single-family housing increased 11 percent in August, following a 9 percent decline in July. Construction of buildings with five or more units rose 6 percent after climbing 28 percent in July, the Commerce Department said.