After five months of indecision about how to aid tenants hard-pressed by the lifting of rent controls, the Montgomery County Council yesterday enacted legislation to provide rent relief to low-income tenants whose housing is not already subsidized.

Under the new program, which will be in effect for two years, about 10.700 tenants with incomes less than $10,000 a year would be eligible for annual subsidies up to $435. The average yearly supplement will be $244 for tenants over 62 and $122 for those under 62, according to revenue office estimates.

Although she voted the legislation as a "minimal first step," Council President Elizabeth Scull said that next Tuesday she still will attempt to introduce legislation reestablishing ceilings on rent increases in the county.

Last December, the County Council by a 5-to-2 vote eliminated rent controls that for four years had limited most rent increases to 6.1 percent.

Since the first of the year, about 37 percent of the country's tenants have received rent increases exceeding the guidelines, in many cases by 20 percent to 50 percent, causing debate among council member about the effectiveness of the voluntary program.

Since rent controls were lifted, the average rent in the country has gone up 12 per cent, raising the average rent to $323, the highest in the metropolitan area, according to the countuhighest in the metropolitan area, according to the county housing office.

Council member John Menke, who is seeking the Democratic nomination for county executive and who sponsored the bill ending rent controls, proposed the rent supplement program to augment the voluntary guidelines, which he contends are working well.

Under the measure, which extends the existing rent relief law for tenants over 62 years old to younger persons at half the benefit, will apply to tenants who have lived in the county at least six months before July 1 of the two years the program will be effective.

In adopting its fiscal 1979 budget in May, the council set aside $2 million for the program, which revenue officials now say will cost an estimated $1.7 million.

County Executive James P.Gleason proposed a similar rent supplement program of his own and "might" sign the council's bill once he reviews its amendments, according to a Gleason aide.

A veto by Gleason apparently would be easily overridden by the council, which approved the measure by a 6-to-0 vote, with council member Dickran Hovsepian absent because of illness.

Only five votes are required to override a veto.