A development group has signed a contract to buy the 121-year-old Central National Bank Building, a decaying landmark on Pennsylvania Avenue NW that was marked for rehabilitation as part of renewal plan for the avenue. The price of the building was not disclosed.

The Historic Central National Bank Redevelopment Group, was formed to buy and refurbish the five-story building, is also negotiating to purchase two adjacent buildings. The top floor of those two buildings once served as a studio for photographer Mathew Brady, the developers said.

Know to many as the Apex building, because the Apex Liquor Store is located on the ground floor, the twin turreted structure at the corner of Seventh Street and Pennsylvania Avenue formerly housed a newspaper, a hotel and the bank. The building's current owners, the Davidson family, also own and operate the Apex store.

The development group, which includes Philadelphia land planner Andrew Kinzler, said it hopes to lease the first floor and basement to a local bank (Central National Bank no longer exists) and convert the remaining floors into office space.

The proposal must be approved by the Pennsylvania Avenue Development Corp. before the sale can be completed and the work begun. The developers expect the building to be ready for occupancy in 1983.

Bruce Burger, an attorney who heads the development group, said that, because of the costs of renovation, the cost of office space in the building will be high.

Harman Cox Architects have been retained for the initial design and feasibility work on the project.

The Historic Central National Bank Redevelopment Group is an affiliate of the Washington-based United States Land Recources Inc.