Tough new laws designed to protect landmark buildings in the District from demolition will soon be sent to the City Council, according to Ben W. Gilbert, director of the city Municipal Planning Office.

"We are proposing revision of the legislation controlling demolitions," Gilbert said in an interview. "If it's contrary to the public interest, there ought to be some method of preventing demolitions from happening."

According to a draft of the bill, obtained by The Post from sources outside the D.C. government, a demolition permit could not be issued until the mayor determines that "issuance of the permit is necessary in the public interest, or that failure to issue a permit will result in undue economic hardship to the owner." In order to prove economic hardship, the owner would be required to submit such specific information as the amount paid for the property, the income from the property and details about offers received for the property.

The bill, called The Historic Landmark and Historic District Protection Act of 1978, would also regulate new construction, subdivisions and alterations in historic areas. The bill provides protection for historic districts that have been nominated, but not listed on the National Register of Historic Places. The provision would protect buildings such as three houses on N Street NW that were almost demolished recently because the area had been nominated, but not yet listed on the register.

At present, District laws provide only for a 180-day cooling off period during which interested parties are supposed to explore alternatives to demolition.

The delay-in-demolition provision, enacted in 1973, is invoked after both the Join Committee on Landmarks and the state historic preservation officer have held public hearings and determined that the demolition is contrary to the public interest. The delay provisions applies only to landmarks on the D.C. Inventory of Historic Sites and to buildings in historic districts on the National Register of Historic Places.

After the 180 days, the owner of the building is free to demolish it, unless he has come to another agreement with other interested parties.

The only exceptions are in Georgetown and in areas immediately adjacent to important federal buildings. In those cases, demolition permits must be reviewed by the Commission of Fine Arts, a federal review board, which can advise the District government not to issue demolition permits. This advice is usually followed.

Community activists say they have been working with the city government to develop the new legislation.

A case currently before the Supreme Court, however, could strike down all such landmark laws - possibly even before the new District laws are acted upon by the council.

The court is considering an appeal by the Penn Central railroad, which wants to demolish part of Grand Central Station in New York City and construct a skyscraper over it. Penn Central contends that New York City landmark laws, which prohibit Penn Central from constructing the proposed building, are unconstitutional because they deprive the company of its property rights.