The National Theatre, entirely renovated and refurbished, will reopen Jan. 18 with the Broadway musical "42nd Street."

The announcement--made jointly yesterday by Bernard Jacobs, president of the Shubert Organization; the National Theatre Corporation and the Pennsylvania Avenue Development Corporation--signals the beginning of a new era for the landmark playhouse, which has been dark since May 2, 1982.

After about $6 million of structural improvements and interior decoration, the National will be in a position to rival the Kennedy Center for glamor and big-draw Broadway shows. The Shubert Organization, which books the theater, also plans to bring a road company of the smash musical "Cats" to the National, probably in May.

Although an earlier timetable had the National opening by November, delays in construction, due in part to the age of the building, have slowed down Quadrangle-Marriott, the development corporation responsible for the approximately $5 million of structural work.

Meanwhile, the Shubert Organization has hired set designer Oliver Smith to supervise the interior decoration of the facility and has advanced a $1 million interest-free loan to cover costs of restoring period luster to the playhouse.

The newly renovated National will boast expanded lobby space on the street level, and the mezzanine off the first balcony will now run the full width of the building. In addition, lobby space has been carved off the second balcony and rest room facilities have been modernized. The walls of the street level lobby will be covered with $40,000 worth of Italian marble, donated by Jovino Marmi SPA, a marble company in Trapani, Italy. Costs of shipping the marble have been borne by a personal donation from Saudi Arabian Ambassador and Mrs. Faisal Alhegelan.

The theater will also have new chandeliers, fixtures, carpeting and curtains, while performers at the National, long known for the rudimentary condition of its backstage facilities, will find themselves housed in a totally reconstructed tier of dressing rooms.

Smith, who designed the sets for such shows as "West Side Story" and "My Fair Lady" and who has been on the advisory board of the Kennedy Center, said recently that the redecorated National would have a "warm, cheerful look," as opposed to the "restrained modern style" of the Kennedy Center's auditoriums.

I'm hoping to introduce a classical feeling into the decoration of the National in keeping with the basic tradition of Washington itself," he said. The color scheme, he added, would probably be "variations of blues, reds, golds and ivories."

Under the complex arrangements governing the National Theatre, the facility is owned by the Pennsylvania Avenue Development Corporation. In exchange for the right to develop the hotel and commercial complex that surrounds the National, Quadrangle-Marriott agreed in 1979 to the structural upkeep of the theater.

The theater's policies, administration and interior maintenance, however, have been handled since 1974 by the nonprofit National Theatre Corp (NTC). In 1980, NTC hired the powerful Shubert Organization to provide bookings, but disputes and personality differences between NTC's board and the Shuberts brought the theater to a point of near paralysis two years later.

However, both Jacobs and John D. Adams, chairman of NTC since last November, recently predicted a harmonious working relationship in the future. "I don't think we've ever had the kind of problems with a theater we had with the National," Jacobs said. "But everyone is very cooperative now and everything seems to be going along fine."

"This theater is going to be a real asset for the city," echoed Adams. "Any disagreements in the past were over policy--certain [NTC] board members were looking at the National from a business standpoint, others were not. But theater is a business and it has to be run that way." He called the Shubert management "exemplary."

The theater's opening will be marked by a black-tie gala the week of Jan. 18, to be chaired by Carol Laxalt, wife of Nevada Sen. Paul Laxalt, and Betty Wright, wife of the House Majority Leader Jim Wright of Texas. However, NTC, desirous of countering the image of a rich man's theater, has launched a corporate fund drive to subsidize low-cost tickets for students and the elderly. Although a price scale for "42nd Street" has not been announced, the last show to play the National before it closed, "The Pirates of Penzance," had a top ticket price of $35. To date, the drive has raised $100,000.

With the powerful Shubert Organization behind it, the National can be expected to offer stiff competition to the Kennedy Center for the big Broadway attractions. The Shuberts own more than 20 theaters in New York and on the road and are also one of the most active producing forces in commercial New York theater. Jacobs said that the production of "Cats," envisioned for May, will be the second road company of the musical, which continues playing to capacity at the Winter Garden in New York. Restaged for proscenium theaters, the musical will begin a countrywide tour here. Among other likely bookings for the National: the musical "Dreamgirls" and Neil Simon's comedy "Brighton Beach Memoirs."

Operating profits from shows playing the National Theatre will be used to pay off the costs of interior decoration--"hopefully in eight to ten years," according to Adams.

The NTC will also revive its series of public service outreach programs, Noon at the National, a children's theater, and a Monday night series, showcasing new talent and scripts. The Gould Corporation, an electronics firm based in Rolling Meadow, Ill., has contributed $10,000 to launch Noon at the National, probably in February. Under NTC's prior leadership, Noon at the National's programs leaned heavily toward politics and current affairs. Adams said it would have a more bipartisan emphasis in the future and would concentrate initially on artistic matters.