The Arlington County board has given final approval to the first phase of construction on the Pentagon City tract, apparently clearing the way for building to begin this spring.

By unanimous vote, the board approved plans for the most residential part of the tract which is being developed privately and will include a major hotel, commercial, office and residential development.

South of the Shirly Highway, west of the Pentagon and north of Crystal City, the tract, which covers 116 acres, is expected to be one of the most profitable developments in the county.

What the board approved Saturday were plans for a section of the tract called Parcel 5. Parcel 5 is nearer to other residential developments than other portions of Pentagon City. It contains a number of projects that the developers, the Cafritz-Tompkins group, had to agree to in exchange for obtaining zoning approval for the land. On that part of the tract will be built 220 townhouses, a high rise with 300 subsidized units for the elderly and a five or six story nursing home.

Another high rise for the elderly, which would not be subsidized, was also originally planned for that parcel. On Saturday, however, the developers sought and gained permission to build that unit on another parcel. The board approved that change despite some citizen protest that the careful planning for the development was beginning to be chipped away.

"We are concerned about the magnitude of the change," said John Marr of the Arlington Ridge Civic Association. He said that, since the developers had found there was not enough room on Parcel 5 to build the high rise and to keep density sufficiently low, he questioned if there was sufficient space on the part of the tract of which the units were being transferred.

Citizens opponents of the development said they are appealing a court decision earlier this month upholding plans for developing the tract.

The court's decision was the latest setback for the Pentagon City coordinating committee, a group of citizens who live near the proposed development and have argued for developing the tract with less density.

As the board accepted the developer's opposed change in the design of Parcel 5, by a 4 to 1 vote, it also agreed, to allow the developer to put the title to 13 acres, which is to be donated to the county for a park, in escrow. The developer's attorneys said they preferred to wait to transfer title to the park until the rezoning litigation is cleared away.

The housing for the elderly project included in the package approved Saturday will be located near the intersection of South Fern Street and 15th Street South. The building will also include a convenient store, a library lounge, green house, craftroom, kitchen and dining facilities, laundry, lounge and social hall.

Both the housing for the elderly and the nursing home will occupy sites of about three acres each.

The nursing home is expected to contain 240 beds, although it could be expanded to 300 beds.

The townhouses are designed as two-story units stacked one on top of the other. The houses, which are expected to sell for between $55,000 and $65,000, will include 78 two-bedroom units and 142 three-bedroom units. Each will have a fireplace and a private concrete patio, balcony or courtyard area. They will be sold as condoniniums and will share access to a swimming pool.