Minority shareholders have sued the National Press Club, alleging that the proposed sale of its 55-year-old National Press Building in downtown Washington is "unconscionable" and illegal.

Club officers say the proceeds of the sale are needed to complete extensive renovation of the building, which will include twice as much space as the press club now occupies and some new luxuries. Under terms of the proposed sale to an investment syndicate, the club would get the building back in 45 years.

The shareholders' suit by TBK Partners, however, says the refinancing plan is too risky and too costly. TBK, a private investment firm in New York, asked the Delaware court that has jurisdiction over the National Press Building Corp. to declare the sale illegal. The suit also asks the court to order the National Press Building Corp. to buy the shares of all minority owners at the fair market value, which TBK estimates to be $3,000 a share, or more.

"A two-story cocktail lounge in the press club quarters paid for by a public company is just not right, and a private elevator and entrance," said William Klein II, attorney for TBK.

The cocktail lounge, private entrance and elevator are part of the press club's renovated quarters, which will cover 58,000 square feet, 30,000 more than it occupies now.

William D. Hickman, president of the building corporation, said "on advice of counsel, I won't be able to talk about" the suit.

Massive cost overruns forced the club, which holds 78 1/2 percent of the stock in the National Press Building Corp., to seek new financing, corporation officers said. The club obtained a $45 million loan commitment last year from a New York state employes pension fund but $75 million is now needed to finish the job, an amount the teachers' fund says it cannot provide.

The National Press Building Corp. plans to use $15 million from the sale of the building to the syndicate and a $60 million loan from Merrill Lynch, Hubbard Inc. to cover the cost. A major attraction for the investors in the Boston and New York-based syndicate, L-G Associates, is a large tax benefit.

Court documents allege that the building corporation would give up $15 million in tax credits and approximately $1.8 million a year in rent that the club's additional 30,000 square feet could be expected to bring in at market rents. If the National Press Building Corp. rented this space at competitive rates, the additional income would enable "the corporation to obtain conventional permanent financing" of the $75 million the club needs to pay for the renovation, the documents said.

The space the press club will occupy "is choice space, comparable to the finest in Washington" and could be expected to bring in an average of $60 a square foot over the 45-year period of the sale, the minority owners allege.

The National Press Club's 78 1/2 percent ownership of National Press Building Corp. amounts to 20,108 shares, and NPBC owns and operates the historic building at 14th and F streets NW. TBK Partners holds 12 percent of the remaining shares, which amounts to 670 shares.