Arlington's County's stock of low-cost rental housing is expected to diminish rapidly in the next few months as two major garden apartment complexes are converted to condominiums or undergo substantial rehabilitation that will result in sizable rent increases.

Arlington housing officials say they are alarmed by the proposed conversions, which will affect an estimated 7,000 tenants.

During a discussion of federal housing subsidies before the Arlington County Board last week, housing services devision chief Edward Brandt leaned forward and said: "As Buckingham is rehabilitated and Claremont goes condominium, there's going to be a lot of pressure on people to find housing. We're talking about a majority of the low- and moderate-income housing stock in Arlington!"

Brandt's concern about the Buckingham and Claremont apartments, two of the country's largest complexes that house many low- and moderate-income tenants, comes in the wake of proposals by their new owner.

Associates of Ohio real estate developer James D. Klingbeil have announced plans to convert the 520-unit Claremont project, on S. Walter Reed Drive, to condominiums. Conversion will begin shortly and by next spring present tenants, many of whom are recently arrived Asians, must find new housing.

Substantial renovation of a large section of the 1,800-unit Buckingham Apartments, on N. Glebe Road, is scheduled to begin in January. Many tenants are Indochinese refugees or long-time elderly residents. Renovations there will result in monthly rent increases of at least $100, according to Klingbeil associate Jerry Vogel.

According to Vogel, the Klingbeil firm also plans to build luxury townhouses on a 27-acre site, one of Arlington's last vacant large tracts, near Claremont.

County housing officials compare the planned conversions with the rehabilitation of Alexandria's Shirley-Duke and Regina Apartments, which has displaced an estimated 1,200 low-income families. The last major conversion in Arlington, which currently has one of the lowest apartment vacancy rates in the metropolitan area, occurred in 1973 when Fairlington Villages was converted to condominiums. That conversion removed 3,000 moderately priced apartments from Arlington's rental housing market.

Last week the county board, in a move which Vogel said he found "stunning," refused to endorse his proposal to use nearly $2 million in federal housing money to rehabilitate the first section of Buckingham. Vogel's proposal would have concentrated 444 units of badly needed subsidized housing in one corner of Buckingham.

Vogel said he had been told by the Department of Housing and Urban Development that because of the agency's budget requirements, the entire $2 million subsidy must be committed before Sept. 30, when the federal fiscal year ends. If this is not done, housing officials warned the county board, Arlington stands to jeopardize $1,2 million in federal community development money.

Because renovations for all sections of the complex could not be planned before the Sept. 30 deadline, Vogel and the county housing staff recommended concentrating the subsidized units in one section.

Board member Joseph S. Wholey said he favored subsidizing 60 percent of the units in that section. An informal survey of tenants taken last week indicated that about 60 percent would qualify for federal rent subsidies.

"We don't think 60 percent (subsidized units) would work," Vogel told the board. "Once a project is labelled as a subsidized project it's difficult (to market)" the non-subsidized units.

Wholey reminded Vogel of an earlier statement to the board that the federal money would be used to scatter 363 units of subsidized housing throughout Buckingham. When Vogel denied saying that, a visibly angered Wholey asked sharply, "Which team are you on?"

Last June the board turned down a competing plan by a Boston developer to build a 275-unit subsidized housing project in South Arlington in favor of Vogel's proposal to scatter subsidized units throughout Buckingham.

Board Chairman John W. Purdy initially favored the concentration of units. "These people are already there so we're not supporting a new concentration," he said. However, Purdy later voted in favor of a measure that, in effect, would scatter the units.

"How would you like to live in that section and go to school with that label?" asked board member Dorothy T. Grotos, who warned that such a move could create public housing ghettos. "(Vogel) is concerned about the market place but I'm concerned about the neighborhood. If we're going back to that old, worn idea of public housing we're crazy."

The board apparently agreed, voting unanimously to support only a 20 percent concentration of subsidized units in the first section.

"I don't talk scattered site and I don't think scattered site," Vogel said this week. "We can't figure out what the county board did. The thing that stuns me is that if they had agreed with us, they would have gotten even more subsidized units. It's hard for me to believe they would turn down their own citizens."

Wholey explained the vote this way: "The board was very interested in establishing the precedent of scattered-site housing. We want them to go forward with what they said they wanted to do . . .

"There is a danger of having a lower and lower proportion of units available to low income residents, but I predict we'll still have a good mix in Arlington. Our emphasis has been on rent supplements which preserve the anonymity of the individual being helped.

"It's not clear to me whether the HUD money is lost. I think we could get funding for 20 percent of the units each year. The next move is up to Vogel."