The Arlington County Board has voted unanimously to sell Xerox Building developer Theodore Gould a parcel of land across the street from his building in Rosslyn for $1 million.

Besides the money, the county gets the right to put a park on the land, sandwitched between Ft. Myer Drive and North Lynn Street, which Gould, in effect, will turn back over to the county for that purpose. In addition, Gould will construct curbs and sidewalks around the land. For his part, Gould gets the right to put additional offices on yet another parcel of land adjacent to the Xerox Building, where he plans to construct a new building.

The transaction, acted on unanimously by the board last week, solves a problem the county has been wrestling with since 1973. The county bought the land in 1969 for the right of way necessary to complete Ft. Myer Drive, Fairfax Drive and North Lynn Street. In 1971, the board approved selling the tract, known as the Hessick tract. They sold it in 1972 to the Pomponio brothers for slightly more than $1 million, but the Pomponio brothers later defaulted.

Parks in the Rosslyn area, where the concrete is dotted with high-rise buildings, are "practically nil" according to Arlington County transportation director H. S. Hulme. The timetable for developing the park may depend on Gould's construction schedule, since the park site may be used for storing construction materials while the office building is going up.

Gould, who also owns two office building on Dupont Circle, has not yet submitted plans for the office building to the county for approval. He told county officials, however, that he expects to begin construction by early next year.

The park site is currently occupied by a warehouse and debris, county officials said.

In other action the county board also voted to raise ceilings for qualifying and some benefits for the county's tax and housing expense relief program.

Under the tax relief program, those who qualify may defer real estate taxes until the house in which they live changes hands, or they may take a partial exemption from property taxes of up to $923 a year (the new figure set by the county board), based on income. The county board increased from $4,000 to $6,000 the income ceiling for qualifying and voted to allow heads of households who are permanently and totally disabled to qualify for the program as well as those over 65 years of age.

On the housing expense relief program, the board increased from $9,000 to $10,000 the maximum income allowable for families of three or more and raised the maximum rent level for the elderly and handicapped to $200 per month. The program, which covers the elderly from 55 to 64 years old, needy working families with children and handicapped persons, pays a subsidy for housing costs on moderate priced housing when those costs exceed 25 per cent of the recipient's income.