The Pension Building, one of Washington's secret landmark delights, would be converted into a National Museum of the Building Arts under legislation proposed yesterday by Sen. Charles McC. Mathias (R-Md.).

At a press conference there, in what the American Institute of Architects' guide to the city calls "the most astonishing room in Washington," Mathias announced a plan to create a showplace for achievements in architecture and the building arts.

The 91-year-old red brick building at 5th and G Streets NW has been used the last six years as an annex for the D. C. Superior Courts. The city government's lease expires June 30, and courts administrator Donald Peyton said yesterday that the 17 civil courts and most of the related offices located there are scheduled to move to the new courts building the first week in May.

Renovation of the structute, which was built in 1885 and served as headquarters for the Federal Pension Bureau until 1926, would cost about $11.6 million, plus $2 million more for preparation and blueprints, according to the Committee for a National Museum of the Building Arts Inc., which developed the idea using several public and private grants. Mounting permanent exhibitions would cost another $7.5 million. The annual operating cost was estimated at $3 million.

In announcing the bill, Mathias said the building "is an ideal setting for the demonstration and study of American contributions in civil engineering, architecture, the building trades and crafts, landscape architecture, city planning and urban design."

Mathias, sitting before an Arthur Hall Smith painting, "Solar Requiem," that is part of an art exhibit on the spacious main floor, called the building "one of the great monuments of 19th-century America," adding that the building "is more expensive than I could possibly be."

Cynthia R. Field, president of the nonprofit committee, said the earliest work could begin on the transformation would be mid-1979 and that "with a miracle," the museum could be completed within three years.

Herbert M. Franklin, secretary-consel for the committee, said planners hope the renovation could make use of "updated energy techniques," noting that the roof "lends itself to solar heating."

Inspired by Renaissance Rome's Palazzo Farnese, the architect, Gen. Montgomery C. Meigs, devoted most of his imagination to the enclosed interior court with its 75-foot-high columns, lined on all four floors with arcaded galleries that would house the museum's permanent exhibits.

The main hall, scene of inaugural balls for presidents from Cleveland to Nixon, would be preserved as open space and for temporary exhibits of tools of the building trades and exhibits.

Ten other senators joined Mathias in co-sponsoring the proposal, which could get to vote in Congress later this year.