The Arlington County Board, conceding it was committed under a previous zoning decision, yesterday reluctantly approved a high-rise rental building for a site it had envisoned for a "life care" facility for the elderly.

Although board members have traditionally embraced developers' plans for rental buildings, several pointedly remarked that they believed that the proposed building near the courthouse did not merit the density approved for the site two years ago.

"The county has gotten into a position we didn't intend to be in," said County Board member Ellen M. Bozman. "We rezoned that property for a life care facility we very much wanted and felt we very much needed."

Despite the board's complaints, County Attorney Charles G. Flinn said local governments in Virginia long have lacked the power to demand that specific projects be built on a site in return for rezoning. If a proposed project fails, the local government cannot reverse its rezoning because it wants only a certain project on the site, he said.

Local governments in Virginia, Flinn added, rarely have succeeded in court challenges to decisions that reduce the amount of development allowed on a particular site.

Board members said nonetheless that they may have to seek legislation from the Virginia legislature that would give them greater power over rezonings.

At the core of yesterday's dispute was a 1983 rezoning the board approved for a life care facility for the elderly that the Temple Foundation planned to build on a two-acre site bordered by Fairfax Drive, Clarendon Boulevard and North Barton and North Cleveland streets.

The facility would have provided a 180-unit retirement home and a 74-bed medical facility for the county's growing elderly populaton in a building no taller than 120 feet. Because board members said the need for the facility was so acute, they rezoned the site for a higher density than its previous designation would have allowed.

The foundation encountered financial problems in 1984 and abandoned the project. Yesterday the Dittmar Co. of Dunn Loring pressed its case for a high-rise rental building on the site.

Its plan initially called for a 284-unit building that would be "terraced down" from 11 to seven stories and would provide 410 parking spaces. It would require a greater density than that given to the life care facility, but it would be within the site's zoning limits.

The board, wanting to provide flexibility for a possible redesign of the project, left undecided yesterday the exact number of units, which will ultimately determine the number of parking spaces, but it set a height limit of 136 feet. Planner Hal Gidden said that would allow no more than 10 stories.

Board member Albert C. Eisenberg was the only one of the five County Board members to dissent yesterday, arguing that the developer should be kept to the slightly smaller density approved for the life care facility. He complained that the projected rents were too high and could be reduced if the developer secured tax-exempt financing.

"We're allowing the developer to get another bite of the apple in return for something less than we thought we were getting," Eisenberg said.