A $140 million office and residential complex was unanimously approved yesterday by the Arlington County Board, one of four major projects at the core of the rapidly redeveloping Ballston area.

In other action, the board unanimously approved a financial agreement that will preserve 200 units of the Lee Gardens apartments as low- and moderate-income housing.

The Ballston development includes a park, which the county has long wanted in the area. The Oliver T. Carr Co. will develop the project.

Plans call for a 14-story office building with retail shops and a 22-story residential tower on a block bounded by Wilson Boulevard, Fairfax Drive, North Stuart and North Taylor streets.

Construction will begin by next spring, said Thomas A. Carr, vice president with the firm. Plans for the one-acre park call for outdoor dining areas, sculpture and fountains.

One unresolved issue is the fate of two parcels of land on the Wilson Boulevard frontage.

Carr said his firm is negotiating with the owners of the parcels and, if successful in buying them, will return to the county board with plans for a larger residential building.

As a condition for approval, the developer agreed to contribute up to $300,000 to build a tunnel from its project to the Ballston Metro station. The firm withdrew a request for added density and was turned down by the board on its proposal that the county pay for maintenance of the park.

On Lee Gardens, the board's action underpins a complicated financial arrangement that will allow the nonprofit Arlington Housing Corporation to buy 364 units in the north section of the complex. Plans call for 200 of those units to be set aside for low-income tenants who qualify for federal rent subsidies. The Virginia Housing Development Authority agreed last month to issue $34 million in bonds that will enable the housing corporation to buy North Lee Gardens.

But both the VHDA and the bank that is guaranteeing payment of the bonds asked that the county guarantee the solvency of the project.

The board did so yesterday by agreeing to a "moral obligation" to step in should the Arlington Housing Corporation run into financial difficulties on the project, and put $1.5 million in reserve.

While not legally binding, failure to honor such a commitment would affect the county's credit rating in the bond markets, County Manager Anton S. Gardner said.

Lee Gardens has been in the news since late last year, when the Artery Organization of Bethesda bought the complex and announced plans for an extensive renovation that is expected to displace about 3,000 tenants, most of them low-income Hispanics.