The Arlington County Board voted yesterday not to raise the real estate tax rate this year, a decision made easier by new revenue forecasts that show the commercial tax base exceeding residential for the first time.

The board will not officially adopt this year's tax rate until March 15, at which time members indicated they may consider a cut in the current rate of 95 cents per $100 assessed value, the lowest in the metropolitan area.

The rate will not be increased because the board agreed to advertise a March 11 hearing on the 95-cent rate, and under Virginia law, the board cannot adopt a rate higher than that advertised.

Since the proposed $288.8 million budget for fiscal 1987 was unveiled three weeks ago, revenue projections based on assessments have been revised to reflect an extra $5.1 million in income that board members said they may want to use to cut taxes, offset expected losses in federal aid or improve county and school programs.

At the School Board's request, the County Board immediately set aside $450,000 that the schools do not expect to spend this year for school programs next year.

Of that $5.1 million increase, nearly $4.5 million is expected to come from revised estimates of real estate tax revenues, and the remainder primarily from unspent funds this fiscal year.

"We still need to be cautious, said board member Albert C. Eisenberg, referring to the new revenue predictions. "While the clouds have parted a little bit, there are uncertainties we've got with the federal budget . . . that could bring those clouds back again."

After years of wrestling with development controversies, the county will see its commercial base outstrip its residential base, 51.4 percent to 48.6 percent, this year for what is believed to be the first time. Arlington counts rental apartment buildings as commercial property.

The county's total tax base is projected to be about $10.3 billion, an increase of 15.4 percent over last year and 3 percent more than the proposed budget envisioned. Of that amount, $442 million, or an unprecedented 4.9 percent, comes from new residential, retail and office construction in the county's high-rise development corridors and the Shirlington area.

Existing commercial structures jumped an average of 18.3 percent, while new growth increased 7.65 percent for a total increase in the commercial base of almost 26 percent -- or $1.1 billion -- over 1985. This year, commercial properties will amount to more than $5.3 billion of the tax base.

It is a figure Board Chairman Mary Margart Whipple called "a phenomenal increase," and that budget director Mark Jinks said "reflects the desire of businesses to locate in Arlington."

Existing single-family homes, which account for nearly 40 percent of the tax base, increased 3.5 percent in value, and new construction was up just under 1 percent, for a total residential increase of 4.4 percent. Single-family homes constitute almost $4.2 billion of the tax base.

Existing condominiums increased 3.25 percent while new condos jumped 10.7 percent. They will account for about $895 million of the tax base.