Arlington's 20,600 cable TV subscribers will pay an extra $1 in monthly service charges next month after the County Board acted against the advice of its Cable Television Advisory Committee yesterday and allowed the Arlington Telecommunications Corp. (ARTEC) to raise its rates.

ARTEC executive vice president John E. Evans said the new $10.95 rate, effective Jan. 1, is needed to help the cable company weather inflation and pay for the added costs of community programming.

Advisory Committee member Joseph Lewis told the board the rate increase was "inappropriate," since it comes only three months after ARTEC won an extension of its franchise until l995. Instead, he recommended that ARTEC delay any increase until it has refinanced its debt next March. Only board member Dorothy Grotos voted against ARTEC's request.

"Certainly, when the prime rate was going up they used that as a factor," she said. "I'm disappointed they're not doing the same when the prime rate is going down."

The board also voted yesterday to seek legislative approval for a series of changes in the form and structure of county government, as recommended in a recent study by a citizens committee.

But a proposal to hold County Board elections every other year appeared unlikely to get the support of Arlington's Democratic legislative delegation, since the board's two Democratic members voted to keep the elections on an annual schedule. Any change in board elections would require approval of county voters.

Other changes endorsed yesterday would give the County Board more influence in the hiring and the firing of department directors. Under the current system board members are forbidden by statute even to discuss personnel matters with the county manager. The proposed change would give board members the right to advise on the selection of department heads.

In addition, the board wants to be given veto power over the county manager's selection of the county attorney. Arlington is the only county in Virginia in which the county attorney is accountable to the manager and not to the board.

The board also approved plans for a major office-residential development on 6.4 acres of county-owned land behind the courthouse, in the middle of Arlington's Metro corridor.

The county will solicit proposals for the Courthouse Plaza on Jan. 1, in anticipation of starting construction in late 1983. Plans call for a minimum of 440 residential units, a maximum of 470,000 square feet of office space, and 60,000 square feet of retail space.