Montgomery County Executive Charles W. Gilchrist called yesterday for a ceiling on government spending next year that would hold the county's 1984-85 budget to about $870 million--increasing it by less than 6 percent over current spending.

In a three-page memorandum to department heads, Gilchrist said budgets for the fiscal year starting July 1 should show increases no higher than the expected annual inflation rate of 4 to 5 percent.

"I know that you share my continuing concern that our budget not place an increased burden on the taxpayer," wrote Gilchrist, who announced the budget cap in a luncheon meeting with reporters.

"At the same time," he added, "we want to continue and enhance programs directed to the less fortunate."

Gilchrist's budget ceiling proposal, which would also place a cap on the size of the county work force, could be amended by the County Council as it debates the budget.

Gilchrist said he intends to expand the county-operated Ride-On bus program by hiring 178 new drivers and mechanics and purchasing 50 buses costing $100,000 each for new routes in Bethesda and Rockville. The addition of those workers and 52 others would have to be offset through employe attrition or by contracting county services to private firms, he said.

"Considering how tight things are now, making this offsetting reduction will be extraordinarily difficult," Gilchrist said.

Gilchrist's proposal hinges on only slight increases in county property tax bills.

If tax bill increases are limited to 5 percent and current revenues do not change, next year's budget would amount to $870 million, or 5.7 percent more than the county's current $822.8 million spending plan. Last year's budget was $783.3 million.

Although Gilchrist took pains to call the departmental budget ceilings "guidelines," aides to the executive said the memo had the effect of an order. Several department heads already have complained privately about the limits; however, some officials, especially those in large county departments such as health, enivronmental protection and police, might be allowed larger increases than others, the aides said.

At his meeting with reporters, Gilchrist appeared adamant about capping county spending. In fact, he repeatedly called for a budget increase of no more than 4 percent--even though his message to department heads allows for a larger increase.

In another development yesterday, Gilchrist renewed his call for opening the Metro Red Line extension to Montgomery by December 1984. "It should have been opened yesterday," he said, adding that Metro officials now say it could be early 1985 before the eight-stop extension is opened.