BRYANT PARK PROJECT APPROVED

An extensive renovation of Bryant Park was approved unanimously yesterday by the City Art Commission. Afterward, the head of the Bryant Park Restoration Corporation said construction might begin as early as spring.

A proposal to build a restaurant on the upper terrace of the park, behind the New York Public Library, is not yet ready for submission and was not considered yesterday by the commission.

The renovation is to be accompanied by the excavation of the central lawn, where a two-level subterranean extension of the library stacks is to be built. Both projects are supported by Parks Commissioner Henry J. Stern, who exercised his privilege to join the commission in its 9-to-0 vote.

The president of the Art Commission, Edward A. Ames, said there ''was a good deal of enthusiasm'' among the panel's members about the ''openness'' of the new plan. More Entrances

Under the plan, the number of entrances would be increased to nine from six, three of the entrances would accommodate the handicapped, a balustrade around the lawn would be opened at four places, hedges around the park would be removed and two ''gatehouses'' would be built at the remodeled entrance at 42d Street and the Avenue of the Americas.

The executive director of the private Bryant Park Restoration Corporation, Daniel A. Biederman, said the vote was ''very encouraging.'' He said construction would ''certainly'' begin this year. Neither he nor Mr. Stern would predict how long the park would be closed to the public or when the project would be completed.

In considering much the same plan last Tuesday, the city's Landmarks Preservation Commission said it could not issue a favorable report on an application that left out what was proposed for the upper terrace and warned that it would consider ''untenable'' any structure on the terrace that impeded views of the library.

Unlike the Art Commission, the landmarks panel serves only as an advisory body in matters dealing with municipal property.