Kaempfer Investment Builders will break ground this fall on a $50 million, 19-story office building for 1001 N. 19th St., the last undeveloped tract available in Rosslyn.

The 250,000-square-foot structure was designed by I. M. Pei and Partners. A total of 235,000 square feet will be available for leasing. Kaempfer contracted with Clark Construction to build the structure. Julien L. Studley is the exclusive leasing agent.

The building will occupy 25,000 of the site's 45,000 square feet of land. The remaining land will be developed as a park for tenants of buildings in the area.

Kaempfer has also purchased a 10,500-square-foot tract of land at 19th and N. Moore streets in Rosslyn, which is zoned for commercial devemopment. In exchange for a transfer of density rights, they will develop the smaller site as a park for $1.2 million, to be dedicated to the county.

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

*United Services Automobile Association, a member-owned insurance and financial service, purchased a 120,000-square-foot building from Lee Sammis Associates Inc. at 1902 Campus Commons Dr., Reston.

*General Electric Space Systems leased 60,000 square feet from Walker and Co. at Parkridge Two in Reston. The terms of the 10-year lease were not specifically disclosed, but is valued at more than $2 million.

*New York, New York restaurant leased 10,000 square feet for 15 years from from Weaver Bros. at Congressional North Shopping Center in Bethesda.

*HCR Inc. leased 9,385 square feet from the Oliver T. Carr Co. at 1133 15th St. NW. The five-year lease has an aggregate value of $844,650.

*Green Thumb Inc., an employment training program for the elderly, leased 9,712 square feet at 5111 Leesburg Pike, Falls Church, from Transax Data.

*Datagraphics Inc. leased 8,567 square feet for five years from Equitec at East West Towers, 4330 East West Hwy., Bethesda.

*Edison Electric Institute leased 7,465 square feet at 1828 L St. NW from 1828 L Street Associates.

*American Psychiatric Association leased 6,378 square feet at 1444 I St. NW from Live Oak Realty.