General Motors Corp. yesterday confirmed a one-year delay in the opening date of its controversial assembly plant in the Poletown section of Detroit and Hamtramck--part of a wider investment slowdown caused by the continuing depression in auto sales.

GM denied charges surfacing in the final hours of Detroit's mayoralty race that plans for the new plant had been killed.

A group of Poletown residents this year had lost a battle against GM and Detroit's city administration to block demolition of some 1,500 homes and stores in the blue-collar, ethnic neighborhood to make room for the new plant. Charges of the project's death seemed to confirm their worst fears.

But a GM spokesman said yesterday "the plant will be built." The $500 million assembly plant is now scheduled to be completed by 1984 in time for assembly of 1985-model-year cars, one year later than scheduled. Construction will continue, although contracts for equipment have been delayed, a GM spokesman said.

The decision is part of a broader delay in scheduled introduction of new models over the next three or four years, GM indicated. The company reported a $468 million loss for the third quarter, dropping its earnings for the year to $237 million, and auto sales in October were disastrously low.

GM said yesterday that it still is committed to a $40 billion investment plan through 1985 but has had to delay some spending until later years. There were no further details.

GM earlier announced cancellation of a planned $500 million assembly plant scheduled for Kansas City, Kan., and an indefinite delay in a major renovation of its Baltimore assembly plant. A decision on replacing the GM assembly plant in Flint, Mich., also has been delayed for a year.

Construction schedules for two other new plants, in Wentzville, Mo., and Orion Township, Mich., have been stretched out by another year, and they won't go into operation until late 1983 or early 1984, a GM spokesman said.

GM currently intends to build front-wheel-drive luxury coupes and sedans, including Eldorados and Sevilles, at the Hamtramck plant, the company said.

It would not elaborate further on its product plans for the plant, but industry analysts believe the delay in the Hamtramck plant opening was necessitated by a GM decision to put off introduction of a new, front-wheel-drive mid-sized car at its Linden, N.J., plant. That plant will continue to build luxury cars through the 1984 model year, it now appears. Thus, the Hamtramck plant won't be needed until then.