The Central Intelligence Agency is proposing a major new building at its woodland headquarters in McLean that would double the size of the agency's marble complex there, the National Capital Planning Commission was told yesterday.

The new building, which would cost at least $100 million, would be the first major addition to the facility since it was formally opened in 1963. It should be completed within the next five years if Congress authorizes and funds it, CIA logistics chief James McDonald told the commission.

The seven-story building would house several floors of computers and some 3,000 additional CIA employes, now scattered in offices around the Washington area. It would be next to the existing seven-story CIA building, the federal planning agency was told, and would be no more visible to the public than is the existing complex, which is encircled by chain-link fences and shielded by a grove of trees.

While the CIA did not put a price tag on the building, a million-square-foot office building such as the agency is planning would cost at least $80 million to $100 million, according to John O'Neill, executive vice president of the Apartment and Office Building Association of Metropolitan Washington. Planning commission officials estimated yesterday that the CIA's proposed structure would cost in excess of $100 million.

The briefing on the new building occurred as the Reagan administration began major cutbacks in nonmilitary spending. "If there's any federal building project here that's going to be funded under this administration, this is it," predicted an official of the General Services Administration, which handles the construction and leasing of federal office space.

The government has followed a policy in recent years in this area of leasing space in private buildings.

Most of the 3,000 CIA employes who would be housed in the new building would be transferred from area offices, many of them in Northern Virginia. The rising costs of leasing office space for CIA employes and "a concern for the security of our installations . . . has raised the priority of this building," McDonald told the planning agency yesterday.

To lessen the traffic impact of 3,000 additional employes, the CIA plans to stagger work hours there and expand car-pooling efforts, McDonald said.

The total number of CIA employes is a classified secret, but the agency was said in 1975 to have about 15,000 employes, most of them apparently at the McLean headquarters.

The CIA's willingness to disclose the new facility contrasts to the secrecy that surrounded construction of the headquarters. Commissioned by President Truman in the 1950s, the agency for years was not identified by signs at its location. Fairfax County land books long listed the site as owned by the Commerce Department.