Montgomery County landlords have kept rent increases below 15 percent in the county's first year of voluntary rent control, a report by the county's Office of Landlord-Tenant Affairs has found.

Mandatory rent control in Montgomery County was abolished last January in favor of voluntary guidelines.

An analysis of 28,660 rent increases in the county from April 1981 to the present shows that most landlords have complied with the voluntary guidelines and kept rent increases below 15 percent.

The fear was that tenants living in apartments whose rent had been controlled would be brought up to market levels in one blow, "that someone paying $200 for an apartment would be paying $400 immediately," said Richard J. Ferrara, executive director of the county's landlord-tenant office. "That simply hasn't been the case."

Rent control in the county began with federal guidelines in 1971. The county took jurisdiction in 1973 and three years later passed legislation ending most mandatory rent control. Last January it extended voluntary guidelines for all rentals.

The voluntary guidelines, which affected rent increases beginning in April 1981, said rental facilities should not raise the rent of units occupied before July 1, 1976, by more than 15 percent. For units rented after July 1, 1976, the guidelines suggested rent increases of no more than 10 percent.

Ferrara said a 1980 survey of county landlords found that 70 percent agreed to comply with the guidelines. But his office's report showed even greater compliance by county landlords in that rent increases were kept below the guidelines in 77 to 87 percent of all apartment rentals.

The report also found that in more than 61 percent of the apartments studied, rents increased by less than 10 percent. The report did not include apartments with fewer than 12 units, government-subsidized and new apartments.

Only 7 percent of the rent increases in the county last year exceeded the voluntary guidelines, the report said. In three cases, those of White Oak Towers, Pickwick Towers and Park Wayne apartments, the report said, the higher rent increases were due to costly renovations, the "extremely depressed" rent levels of some apartments, and the fact that some of the buildings were newly purchased at high interest rates.

But the report also singled out five major apartment complexes that "neither respected the guidelines nor provided any justification for exceeding them." Those were Colesville Towers/Cole Spring Plaza, Rollingwood Apartments, Manchester Manor and The Falkland Apartments, all in Silver Spring.

A study of monthly rent increase records kept by the county shows that Colesville Towers, for example, increased rent by more than 15 percent in more than 100 of 192 cases. Rent increases up to 46.9 percent were listed for the Rollingwood apartments and as much as 26 percent for Manchester Manor, the records showed.

The Falkland Apartments instituted 78 rent increases for December 1981 ranging from 16 to 34 percent, county records show. Some of the administrators of those apartment complexes defended the increases, blaming them on higher costs, and said they kept the county and tenants apprised of the reasons for the rent increases.

Mike Fisher, a vice president at B.F. Saul, agents for The Falkland Apartments, disputed the county's figures, saying that less than 18 percent of their tenants received rent increases above the guidelines. He cited increased fuel and maintenance costs and "a 99.6 percent increase" in the apartments' real estate tax assessments since December 1979.

Michael Gilmore, property manager for Colesville Towers, said, "I told the county that we would raise the rents slowly if they could guarantee that we would not eventually go back to rent control and they couldn't guarantee it."

Council vice president Michael Gudis, who has been on both sides of the rent control issue, agreed with the report's conclusion that the voluntary guidelines have succeeded. "I was concerned that senior citizens would not be able to afford the rent increases," Gudis said. "But the county supplements rent payments as much as $75 for those who need it."

Gudis was referring to 911 low-income tenants who received rent supplements of $21 to $75 last year if their rent increased by more than 10 percent.