The District of Columbia's historic preservation officer conceded yesterday that an "improper visit" to the shabby Georgetown waterfront in August played a role in her now rescinded Sept. 12 decision approving the construction of a $154 million residential and commercial complex along the river.

Carol B. Thompson, the city's agent for historic preservation, said that after her Aug. 7 visit to the river front she "determined that the proposed project would not block Georgetown's view" of the river and that the heights of the four proposed buildings "would not overshadow the historic district."

The 67-foot heights of the proposed buildings and their size are central to the contention by the Citizens Association of Georgetown and other civic groups that new development is a severe intrusion into the surrounding historic district and should therefore be rejected.

Thompson rescinded her decision favoring the project's developer, Western Development Co., after lawyers for the citizens' group disclosed two weeks ago that Thompson had failed to notify them and the developers of her visit to the waterfront. Yesterday, Thompson, accompanied by members of the citizens' association, its attorneys, development officials and their attorneys and architect Arthur Cotton Moore, officially toured the site for 20 minutes and then held a new hearing.

Thompson said she would issue another decision Oct. 20.

Until she rescinded her earlier decision, it removed the last major barrier to the development of the river front. She ordered yesterday's hearing so both sides could discuss the "relevance and materiality" of the improper visit.

At the 75-minute hearing, David Bonderman, a Georgetown resident and citizens' association attorney, asked Thompson to disqualify herself, claiming the visit had played "a significant role in her decision" and she had failed to answer many questions surrounding it.

Thompson rejected Bonderman's request. The guidelines that govern such hearings required Thompson to notify both sides of her visit and let them accompany her.

Whayne S. Quin, a lawyer for Western Development, which wants to build four buildings on the disputed six-acre tract located between K Street NW and the Potomac River, contended the visit was "repetitious" of facts introduced earlier.