The Arlington County Board has unanimously defeated a request by a multimillionaire New York developer to convert a deteriorating Rosslyn highrise into a luxury apartment hotel.

At last Saturday's often emotional two-hour public hearing, residents of the Arlington Towers apartment complex urged the board to reject the proposal by Manhattan developer Harry A. Helmsley to convert one of four buildings in the complex to a hotel that would bear his name.

Helmsley's attorney, Martin D. Walsh, told the board that such a conversion would reverse the deterioration of the 25-year-old red brick complex overlooking Rte. 50. Helmsley has the controlling interest in a partnership that bought the complex more than 10 years ago.

Once a temporary residence for military and State Department personnel, in recent years, the moderately priced apartments have attracted large numbers of newly arrived Arabs, Asians and Hispanics.

The controversy surrounding the proposed conversion mirrors a similar furor in the District, where scarce rental housing has been converted to expensive condominiums or luxury apartment hotels.

Approval of the conversion would have meant the displacement of 363 residents of the complex's 1,700 units. Hlemsley's attorney Walsh did not say what the owners planned for the other three buildings. Approval of the request also would have conflicted with a long-standing county policy of maintaining residential buildings in Rosslyn, primarily an area of highrise offices with fewer than 4,000 residential units.

"If they're incapable of running an apartment, I don't see that they're capable of running a luxury hotel," said Dr. O. M. Sills, one of 10 Arlington Towers residents who complained to the board of poor maintenance at the complex, including a lack of hot water, broken elevators and trash in the halls.

"We have rats, bugs and seven people living in an efficiency," she added.

For several years, county housing officials have said that Helmsley has refused to answer repeated requests from the county and apartment residents to discuss conditions at the complex.

Helmsley's attorney Walsk acknowledged that "there have been problems in the past and there are problems now."

"This is not a viable rental project and the only thing (Helmsley) finds practical is conversion to a hotel," Walsh said.

However, County Board members were not convinced that conversion was the answer.

"We've been hearing similar complaints about Arlington Towers for the last couple of years," said board member John W. Purdy. "I don't think that his owner has come to us with clean hands and I don't see why we should approve this."