Vacancy rates in office buildings across the United States dropped slightly in the second quarter of 1988, while the availability of industrial buildings increased, a real estate firm reported last week.

The office vacancy rate for metropolitan areas - downtown and suburban markets combined - dropped 0.6 percent to 19.8 percent, extending a two-year decline to the lowest level in three years, according to an analysis by Coldwell Banker, one of the nation's largest real estate companies.Downtown office building vacancies held steady at 16.3 percent.

"The office market continues to improve, while the rise in the industrial vacancy rate is attributable to space being added to the market in excess of absorption," said Gary J. Beban, president of the company's Commercial Real Estate Services.

He said more than 29.2 million square feet of industrial space came on the market during the second quarter, about 23 percent of it in new or otherwise never-occupied facilities.

That added availability brought the national industrial vacancy index to 5.9 percent, its second consecutive quarterly increase.

Ten of the 13 largest markets reported increased industrial vacancy rates and only three - Houston, Kansas City and Southern California - had decreases. The largest increase was in Long Island, N.Y., up 1.4 percent to 4.4 percent.

Coldwell spokesman Raymond G. Torto said the improvement in the office sector was concentrated in the suburban markets, where the national average vacancy rate dropped 0.8 percent to 21.5 percent, the lowest since September 1985.

The largest decline was in mid-New Jersey, where vacancy rates dropped 5.6 percent to 31.2 percent.

In the downtown office vacancy rate category, Honolulu experienced the greatest decline with a 3.3 percent drop to 5.3 percent.

The highest downtown vacancy rate was in Austin, Texas, 37.9 percent, and Oklahoma City, 35 percent.