Construction is under way on an office building in Washington's West End that will turn the facade of a 1930s garage into a contemporary eight-story office building, according a spokesman for the developer, the Kaempfer Co.

The building, being built by Omni Construction Inc., is located on 1250 24th St., on the site of the original B & W Garage. Completion is scheduled for mid-1987.

"We have retained the original front of the garage because of its compatibility with the neighborhood, and have added the comtemporary design as a complement to it. Together, it makes an exciting meshing of old and new architecture," said John Tyers, director of marketing at Kaempfer.

The building will have 230,000 square feet of leasable space, 17,000 of which is for retail clients.

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

*The World Center Limited Partnership from Riggs Bank bought the World Center Building at 918 16th St. NW. for $ 12.5 million.

*The Boston-based development firm Cabot, Cabot & Forbes bought 35.1 acres of land in Loudoun Tech Center from Radnor/Loudoun Corp. for $5.9 million.

*Metier Management, a computer sales and training company, has leased 16,711 square feet of space in the Mark Center Office Park in Alexandria. The five-year lease has a value of $1.7 million.

*Giltspur Expo-Industries has leased 88,000 square feet of space in a renovated warehouse in Jessup, Md.

*Arata Exposition Inc., a convention-display company, has leased 20,222 square feet of industrial space at 15922-30 Tournament Dr. in Gaithersburg.

*Dynatech Laboratories Inc. and Airborne Freight Corp. have leased half of the available space in Sullyfield Commerce Center in Fairfax. Dynatech Laboratories has leased 50,500 square feet and Airborne Freight has leased 27,000 square feet.

*Omni Construction Inc. has completed construction of the 15-story Democracy Center for Boston Properties of Washington. The project is located on Rockledge Drive in Bethesda.