The Kaempfer Co., a Washington-based real estate development and management company, has announced it will develop a site at 1201 New York Ave. NW.

Proposed for the site is a 13-story building containing approximately 314,400 square feet of office space and featuring a central atrium. The project is expected to break ground next month.

The property is located on Washington's East End, an area that has recently been a focus of downtown revitalization efforts.

Other recent developments in commercial realty in the Washington metropolitan area:

*The Potomac River Co. has broken ground on Pitt Street Centre, a complex of three Georgian office buildings located in the north waterfront area of Old Town, Alexandria.

The buildings will be 16,000, 31,000 and 47,000 square feet each. Rust, Orling and Neale is the project architect, with Keyser Construction serving as general contractor. The first two buildings are scheduled for completion next summer.

*Group Hospitalization and Medical Services Inc. has signed a 10-year lease for 149,357 square feet at 500 North Capital St. NW., Smithy Braedon has announced.

Group Hospitalization will occupy seven floors of the newly renovated eight-story office building, which is owned by N. Capital Ltd. Partnership. Occupancy is scheduled for this November.

*Work has begun on the Richardson Center, a three-story office center located at Priest Bridge Drive and Espey Court in Crofton, Md.

Project architect is Grimm and Parker, with design and construction by Gardiner and Gardiner Inc. Occupancy is scheduled for next spring.

*The Fairfax County Department of Social Services has leased the entire 52,000-square-foot Providence Hill Phase II Professional Building, located at Layton Hill Drive and Route 237 in Fairfax City. Hersand Builders Inc. made the announcement.

*Spaulding and Slye announced the following leasing transactions: SMS Data Products Group Inc., a computer company, has signed a three-year lease for 31,000 square feet at Sunrise Technology Park, 12379 Sunrise Valley Dr., Reston; J. G. Van Dyke and Associates, a computer consulting firm, has signed a five-year lease for 11,509 square feet at Two Rockledge Center, located at 6701 Rockledge Dr., in Bethesda.