Montgomery County Executive Charles W. Gilchrist vetoed a bill yesterday that would have extended rent control for two years, and by doing so released about 15,600 apartments from rent control at midnight on jan. 31.

Gilchrist said in vetoing the bill that he does not believe decontrol will result in sharp rent increases, but would instead lead to more rental construction and fewer condominium conversions.

"I am convinced that our history of controlling rents in this county for nearly 10 years has contributed substantially to our condominium conversion boom," Gilchrist said.

The County Council could override the veto with five votes, but council member Rose Crenca, a proponent of rent control, said she is certain the bill does not have that support since it was passed with a 4-3 vote last Tuesday instead of a 5-2 vote that would have given it "emergency legislation" status.

Crenca said she will ask the council at its next session to vote to defer an override vote on the bill. By waiting until "some date in the distant future," council member could monitor rent increases and build a case for rent control, she said.

"If we can build a case that increases will be above the 10 percent Mr. Gilchrist said we can expect, we can show that tenants really are getting it socked to them," she said. Voting to defer action also might have the effct of discouraging landlords from raising rents, said Crenca.

Montgomery County's rent control law, the last among the suburban counties, limits rent increases of controlled units to 10 percent yearly unless the landlord can demonstrate extraordinary costs. About 40 percent of the county's 39,000 units are covred by the law. The bill vetoed yesterday replaced the 10 percent cutoff with a figure linked to the consumer price index.

Gilchrist cited statistics showing that rent increases for controlled apartments averaged 7.4 percent compared to 8.9 percent for decontrolled units, and said he has obtained voluntary pledges from landlords owning 70 percent of the rental units in the county that they would keep increases to 10 percent overall, and 15 percent for formerly controlled apartments.

County law that remains in effect prohibits rent increases for all rental units to be made more than once a year.

Gilchrist said rent control ran counter to efforts to encourage apartment construction. "The county government simply cannot approach landlords with incentive to remain in the rental housing business and at the same time maintain the rent control program," he said.

Gilchrist said the county is building up a fund based on the 4 percent tax on the sale of condominiums to be used to assist rental housing construction and elderly tenants dislocated by condominium conversions.

Gilchrist emphasized that the majority of apartment dwellers in the county is not currently under rent control.

"Many tenants who have contacted my office to express concern about the termination of rent controls have been shocked to learn that they have been decontrolled for several years and would not be helped by this bill anyway," Gilchrist said.