The District's historic preservation officer rescinded yesterday the order she had handed down two weeks ago approving construction of a $154 million residential and commercial development along the Georgetown waterfront after she acknowledged that she had made an improper visit to the site.

Carol B. Thompson, the city's agent for historic preservation, visited the waterfront the afternoon of Aug. 7, two weeks after she had closed the hearing record in the controversial case that has pitted the project's proposed developers against the Citizens Association of Georgetown. The association has waged a long battle against the kind of development the proposal outlined.

The visit was improper because Thompson failed to notify the citizens group and the developers of it, as is required by guidelines that govern conduct of such hearings, according to the District Corporation Counsel's Office.

In her decision to rescind her original order, handed down Sept. 12, Thompson said yesterday she would revisit the waterfront the morning of Oct. 5, then convene another hearing at which both sides could discuss the "relevance and materiality" of the earlier visit. It is thought that Thompson will rule again in favor of the development plan, although she could not be reached yesterday for comment.

It was reported that the purpose of her earlier visit to the site was to make a personal inspection of it before ruling.

Yesterday's action, which came the day before developers would have gained the right to obtain building permits for construction, is the latest development in the 20-year feud that has involved the Congress, Georgetown residents, federal agencies and the city government in determining the waterfront's future.

Thompson's visit came to light when citizens groups opposing the new development appealed her Sept. 12 decision that removed the last major barrier to the new construction. The citizens asked both the D.C. Court of Appeals and Thompson to delay the issuance of building permits pending the court's decision on Thompson's ruling.

"We heard from a number of people that she had visited" the site, said Douglas A. Dworkin, of Arnold and Porter, an attorney for the citizens groups, who would give no further details.

The citizens contended in their appeal that the visit violated a 1972 ruling by the appeals court in another case involving the Citizens Association of Georgetown.

The citizens called Thompson's visit "especially troublesome" because it "could well have a direct bearing on many of the disputed issues in this proceeding -- including the appearance of the proposed project, the character of the historic district and the extent to which the project would interfere with river view from other points in Georgetown."

Thompson had ruled that the four proposed buildings would conform with the height, style, texture, color, building materials, arrangement and appearance found in the Georgetown historic district to the north and did not damage the character of the historic neighborhood, as citizens had contended.

The new construction, proposed by the Western Development Co., of Georgetown, would be built on six acres between the Whitehurst Freeway and the Potomac River, an area now occupied by parking lots for cars stored in traffic-violation cases, and for city garbage trucks; a concrete plant and a pile of salt for use during winter storms. The citizens want the site converted into a park.