The U.S. post office at 20th and M streets NW soon will be demolished to make way for construction of a nine-story office building on one of the last undeveloped corners in the prestigious business district west of Connecticut Avenue.

This weekend the window services that have been available at the old building, such as stamp sales and bulk mailing, will move a block south to the Vanguard Building at 1111 20th St. NW.

Developers Ulysses G. (Blackie) Auger, a well-known restaurateur, and his partner Richard Cohen, who own several downtown buildings including a part interest in the Mayflower Hotel, are constructing a new post office building on Ward Place NW, just two blocks west of the old building, to house a mail distribution center and financial offices. Those operations will move in the spring.

"Customers will see no break in their service," said George Conrad, public affairs officer for D.C. post offices. "The last day of service at the old building 1216 20th St. NW will be Saturday, Feb. 16th. On Tuesday, Feb. 19th, the windows will open in the new building" at 20th and L streets, he said. Hours will remain the same.

While office space remains vacant east of 15th Street NW, nearly 4.5 million square feet of new office space is under construction in the city, with nearly half of that amount going up between Connecticut Avenue and Georgetown, according to real estate industry sources.

Last year, developers began constructing new smaller-scale office buildings in the middle of blocks west of Connecticut after nearly all the corner locations had been redeveloped with new buildings.

The area west of Connecticut Avenue to Georgetown and from I Street north to Dupont Circle is known to the real estate industry as the "Golden Triangle" because it commands the highest office rents in the city. Rents in new buildings are now in the $30-a-square-foot range, with some months of free rent and increased tenant improvements such as upgraded carpeting and paneling, real estate sources said.

Auger and Cohen have owned part of the land at 20th and M streets NW for several years, and they recently acquired the last piece they needed to make the site nearly three-fourths of an acre, Cohen said.

The corner now houses a parking lot, run by Colonial Parking Inc., and the post office building, which was built in 1923. The Black Horse Tavern, which is next to the post office building and is owned by Blackie Auger's son, Gregory Auger, will stay open and is not part of the new development.

Russell Lindner, deputy corporate counsel of Colonial Parking and son of Thaddeus Lindner, head of the parking company, said he is not sure how long the parking lot will remain open. "We are simply operating it on the part of the property owners, Mr. Cohen and Mr. Auger. They haven't said when we'll have to vacate it," he said.

Official notices of the relocation will be posted this week, said D.C. Post Office spokesman Conrad.

Representatives of Advisory Neighborhood Commission (ANC) 2B, which includes the 20th and M streets area, said they were unopposed to the new development.

"The address of the new building will be 2001 M Street," Cohen said. "It will be as modern as the address indicates," he said. Construction is scheduled to begin this summer.