The Mayflower Hotel has applied to the District to restrict traffic on adjacent DeSales Street NW and to permit the hotel to build a huge year-round sidewalk cafe on a portion of a new sidewalk -- despite present city regulations which make enclosed sidewalk cafes illegal.

If the proposed 171-foot-long, 13-foot-wide, glassed-in cafe is permitted, it would be by far the largest outdoor cafe and apparently the first private, year-round building constructed on public sidewalk space in the nation's capital.

The improvements proposed for the block-long DeSales Street, which runs between Connecticut Avenue and 17th Street NW along the north side of the 57-year-old landmark hotel, would be paid for by the Mayflower's owners as part of the hotel's $45 million renovation.

"We're proposing to make DeSales an interesting, animated street . . . for people, not a glorified alley" and shortcut for automobiles turning off Connecticut Avenue, said Kingdon Gould Jr., former ambassador to the Netherlands and prime mover behind the hotel restoration. The plans call for narrowing DeSales Street and widening sidewalks at both ends, and landscaping it with trees and benches.

The proposed sidewalk cafe, which Gould claims would resemble the Cafe de la Paix in Paris, would seat about 85 people and open to an indoor restaurant and second-floor cocktail lounge. It would provide a 24-hour restaurant, which the hotel doesn't have now, says Bill Scott, of the architectural firm of Vlastimil Koubek, which is overseeing the hotel restoration.

"It would be like nothing in Washington, not like those wood-and-plastic sidewalk shacks enclosed cafes people are complaining about now," Scott said.

More than a dozen enclosed cafes have gone up on Washington sidewalks in the past three years. District officials claim they all are illegal under the 1961 city regulation that allows open outdoor cafes under certain circumstances but says nothing about enclosed cafes. The city now has more than 100 outdoor cafes, and permits are pending for two dozen more.

In 1977, the city attempted to close down the first of the enclosed sidewalk restaurants: the Paradise Cafe, now Rumors, at 1900 M St. NW. But a D.C. court held the cafe regulation was too vague to be enforced.

Since then city officials generally have ignored the issue, until City Council member David A. Clarke (D-Ward 1) this summer introduced legislation to legalize and regulate enclosed cafes. While many public and private agencies have praised the addition of outdoor cafes to Washington, many, including the National Capital Planning Commission, the Committee of 100 on the Federal City and the American Institute of Architects, are opposing Clarke's bill in its present form.

The Mayflower's lawyers are attempting to ignore the 1961 sidewalk-cafe regulation and have applied to the city under the 1934 public space law that permits leasing of sidewalks and other public land for temporary buildings -- the law that is the basis of the 1961 cafe regulation.

Louis P. Robbins, a former deputy city corporation counsel who, ironically, tried and lost the city's case against the Paradise Cafe, is now arguing on behalf of the Mayflower for the firm of Wilkes and Artis. Robbins said this week he believes District officials can approve the Mayflower sidewalk cafe simply as a use of public space, in effect sidestepping the 1961 sidewalk cafe regulation.

Robbins admits the proposed Mayflower sidewalk cafe could not meet the requirements of Clarke's enclosed-sidewalk-cafe bill and might have to be changed if it is passed. The Mayflower proposal will come before the city's Public Space Committee, composed of the heads of half a dozen District agencies, within the next month or two after the agencies have commented on it. So far none has opposed the Mayflower cafe. The committee is empowered under the 1934 law to make final decisions on use of public space.

The large glass and metal-frame structure would be a "temporary" structure only in the sense that it could be taken down, with some difficulty and expense, if it had to be, Robbins admits.

"We've already got concept approval from the Joint Committee on Landmarks," which must approve changes to historic Washington structures like the Mayflower, Robbins said. "And we've got the approval of all the businesses on DeSales," which include the French restaurant Chez Camille, Raleigh's clothing store, and several office and garage buildings, he added.

Several city officials praised the Mayflower's owners for their direct approach. The owners didn't apply for an open-air sidewalk cafe and then "sneakily" plan to enclose it, said one official, who said "This is actually our first application for an enclosed sidewalk cafe."