Representatives of eight motion picture unions and a major producer today announced an unprecedented cost-saving agreement to produce a major movie here, a plan whose repercussions industry and city officials say could help New York City's film business take it from the top.

The agreement between Robert Evans, president of Totally Independent Inc., and the eight unions means that Evans' $20 million film, "Cotton Club," will be produced here and not in London, as Evans said he had planned.

The heart of the agreement calls for the craft unions to work an eight-hour day, from 11 a.m. to 7 p.m. In almost all cases, film industry labor agreements call for 12-hour work days, including four hours of overtime.

Although New York City has been the site of a substantial amount of on-location filming in recent years, only about one movie a year is totally produced here. In 1981, the city took in about $750 million from film and television productions, according to its Office of Film, Theatre and Broadcasting.

The costs and complexity of dealing with New York-based labor unions is the reason most often cited for filming in California. Even Hollywood studios have been losing business to lower-cost European production facilities, film executives maintain.

The Evans film, a look at the development of jazz during the 1920s tentatively starring Richard Gere and dancer Gregory Hines, will be filmed in Harlem and at the independent Astoria Studios. Filming will begin in July and will take three working months. The studio is owned by a limited partnership of local real estate agents and other business interests.

The union pact was announced at a press conference by Evans, one-time Paramount Pictures chief and now an independent producer, and by Michael W. Proscia, international vice president of the International Alliance of Theatrical Stage Employees & Moving Picture Operators.

Evans said the agreement to film on a so-called "European" schedule would save him $3 million to $4 million. "In this case, unions and management are getting together before there is a disaster," he said, warning of the drift of filmmaking overseas. Proscia said the film would employ 300 to 400 union members.

The other unions involved are the Screen Actors Guild, and unions representing directors, hairstylists, photographers, wardrobe and scene designers, script supervisors and editors.