The Investment Building, a 50-year-old landmark in downtown real estate, has been sold for a reported $12 million to a Washington development group that plans a $23-million total renovation, adding a 12th floor to the classically designed building.

Alan Kay, a partner in the development firm of Rozansky and Kay, said yesterday that his group has bought the building at 15th and KNW from a group headed by investor Sidney Teplin. "We plan a complete floor-by-floor upgrading of the entire structure, beginning late this year, to add 75,000 square feet to the existing 250,000," Kay said.

Kay added that a long-term ground lease is in effect for the 33,000 square-foot site of the building, owned by the New York State Employees Retirement Fund.

Kay also said the Edmund W. Dreyfuss architectural office will supervise renovation and additional floor construction with instruction to "clean up and refurbish the stone facade."

Built by the F. H. Smith Co. in the late 1920s, the building once was the largest downtown office structure. It has had a large number of real estate related firms as tenants over the years. The Washington Board of Realtors has been a tenant for more than 25 years. Widely known attorney Thomas (The Cork) Corcoran, a legendary confidant of Franklin D. Roosevelt, also has his office there. A drug store has occupied the first floor corner for many years and a jewelry store has had a long tenancy.

J. H. DeSibour was the architect for the original building and the plans were registered in 1923. The exterior is limestone with terracotta trim.

Kay said the building now has about 170 tenants. Earlier it had more than 480 individual offices, but many tenants expanded their spaces over the years.

Nearly 20 years ago an earlier owner, Charles A. Gambrill, renovated the building, which still has no central air conditioning. That will be added in the renovation. Gambrill and attorney William T. Hannan headed a group that bought the building for $5 million.

In 1949, Continental Life Insurance Co., bought the building and changed its name to the Continental. W. R. Kelley, head of a mortgage banking firm, bought it for $4.5 million in 1950 from Southland Life Insurance Co., of Texas and restored the original name.

In recent years the ownership has changed several times before the sale to Rozansky and Kay, which also bought and will renovate the smaller, 50-year-old Walker Building at 734 15th St. NW for $3 million.

Developers of apartment and office building projects in this area for more than a decade, Rozansky and Kay has developed the large Greenbriar apartment-condominium complex near Greenbelt.

The firm also has more than $120 million worth of new office and commercial spaces under way, including structures at Tysons Corner, Rockville and Bethesda. The group also plans to develop two more buildings at Tysons, two on the Montrose Road corridor in Montgomery County and another in Fairfax County. CAPTION: Picture, The Investment Building at 15th and K, which has been sold and is to be renovated. By John MacDonnell-The Washington Post