The Arlington County Board early today declared a section of Colonial Village, the nation's first federally insured garden apartment project, a historic landmark.

That designation means that Mobil Corp., owner of the 1,100-unit red brick complex will be barred from altering the exteriors of buildings containing 267 units without a special permit.

The moderate priced apartments could be converted to expensive condominiums without any permit.

The section given the historic designation lies within the area bouunded by Queens Lane, N. Rhodes Street, N. Troy Street and Key Boulevard.

The designation, supported by tenant groups, and opposed by Mobil officials, was granted after an oftenemotional, three-hour public hearing at the County courthouse attended by more than 100 tenants.

Many of those who crowded into the board room were elderly, long-time residents of the complex, who said they feared Mobil would tear down the buildings and replace them with high rises, whose rents would be beyond their means.

"We hope you will preserve all of this environment, which is pleasant and important to the heritage of Arlington and the state of Virginia," said Emma Jean Robinson, a tenant since 1954.

Robinson was one of more than 30 Colonial Village tenants who testified at the hearing in favor of the historic designation for the 43-year-old complex, midway between Rosslyn and Clarendon.

In a strongly worded statement, Mobil attorney Barnes Lawson asked the board to postpone any decision for a year to give the corporation time to devise a plan for the entire complex.

Designating the entire complex "would be so onerous as to be confiscatory," Lawson said. "We may be in the minority tonight, but we happen to be the owners of the property. Let us work with the residents and county staff to come up with a plan," he urged.

In designating a portion of the complex this morning as a historical site, the board also asked Mobil to return with a comprehensive plan for the remainder within a year.

Mobil announced plans last month to preserve 585 units as low-cost rental housing for at least five years. Mobil officials also said any redevelopment is unlikely before 1980.

The 55-acre complex contains more than 12 acres of undeveloped land, near the site of the future Courthouse Metro station.

While county officials have said they favor preserving the county's dwindling stock of inexpensive garden apartments they have also supported high-rise development around the county's Metro stations.