Standing in the historic True Reformer building on U Street NW, where newly installed 12-foot windows have replaced plywood on the second floor great hall, Mayor Anthony A. Williams said the words that District preservationists have waited to hear for a decade.

"Right here and now, we have put an end to demolition by neglect," he said to loud and sustained applause of 300 people attending the annual meeting of the D.C. Preservation League last week. "We have a very important job to do here."

Washington has some of the most stringent preservation regulations in the country, and some of the largest historic districts. But property owners unable to demolish a building because of its historic designation had been able to sidestep the rules by simply letting the building deteriorate. For years, the city has not enforced the existing regulations, allowing dozens of significant structures to crumble.

Williams (D) announced the change in city policy at a meeting called to publicize the league's 1999 list of the 10 most endangered historic sites in Washington, many of which fit the "demolition by neglect" category.

The league's issues administrator, Rich Busch, was among those applauding the mayor's announcement.

"This is extremely important to the city," he said in an interview this week. "It's the first time an official at such an extremely high level has acknowledged that demolition by neglect has occurred and understands the importance of preventing it."

Busch said, however, that the true test of Williams's resolve will come when the District government decides to take action against itself and enforce the regulations on city-owned property.

Department of Consumer and Regulatory Affairs spokeswoman Jacqueline Wallace said the new enforcement will come when former housing inspectors, now called neighborhood stabilization officers, finish their training in preservation issues on July 12. The 28 officers each will be assigned to a Police Service Area, making them readily available to residents, Wallace said.

"The inspectors will be out looking for problems in their assigned neighborhoods," Wallace said--a change from past practice when they only responded to complaints.

"They will be able to take action on housing issues," she said.

For the first time, the city's Historic Preservation Review Board, which is supposed to safeguard historic structures and regulate their use, will have its own inspector on staff, Williams said. He said he is conducting a careful search to find a candidate "who we can be very proud of."

Wallace said the mayor had not made his selection for that job.

The 96-year-old True Reformer building, where the meeting was held, is under restoration by the owners, J.J. Development Inc. of Maryland. Continually occupied by a variety of organizations and commercial enterprises, the national landmark building was never on the endangered list. Its historic facade, however, had long been neglected and is now being restored.

League Vice President Charles I. Cassell announced the third annual list of the most endangered places in the city, which is intended to draw attention to the possible loss of valuable resources.

The list includes five buildings, the entire Anacostia Historic District, all historic D.C. public schools, all pre-World War II D.C. firehouses and all of the city's historic theaters. The final listing is for the Anacostia River Basin, which Cassell described as extending from Northeast to Greenleaf Point, at the southern end of Fort McNair.

The list includes new listings as well as sites from previous years. To qualify for the list, a site must be a resource having historical, architectural or cultural significance that is in danger of demolition, substantial alteration or deterioration through neglect or vandalism, according to the league.