The Rockville City Council is considering a plan to help young families find affordable rental housing in the city, and at the same time provide an opportunity for older homeowners to increase their incomes.

Under the plan, a house could be subdivided into two dwellings -- one for the owners, usually an older couple whose grown children have left home, and a second unit for a younger family.

Council members say they do not know if legal action or zoning changes would have to be taken to implement the plan, or exactly what the city's role would be in carrying it out. The council instructed the city staff to present a written report on these questions in three weeks.

Councilman John R. Freeland, a proponent of the plan, said at a city council meeting this week that young families are forced to leave Rockville because affordable rental housing is scarce.

The plan, he said, would have a two-fold effect. Besides adding to the city's shrinking rental housing stock, the rents could help subsidize older homeowners who often must sell their property because of escalating utility and maintenance costs.

Census figures show the average number of persons per dwelling in the city has decreased. In 1970, according to James M. Davis, the city's chief planner, there was an average of 3.57 people per household. Newest 1980 census figures show that number had dropped to 3.0.

Many homes once occupied by large families now have only two occupants, Freeland noted. "We won't be increasing density in our neighborhoods," he said. "The neighborhoods were built to accommodate large numbers of people."

Freeland said his interest in the plan was intensified recently when his newly married daughter and son-in-law, Sue and Bob Wolfe, moved out of Rockville because they were unable to find housing they could afford.

"We don't want young people leaving the city to find housing," he said."We want to provide adequate, reasonable housing within our city limits."

He said the city would assist homeowners interested in converting their houses into apartments at little cost, since blueprints of all houses built in Rockville over the last 20 years are on file in City Hall.

Councilman John Tyner, who is skeptical about the plan, told Freeland, "A constituency (in support of the housing plan) will be harder to build than you think" because of the possibility it might increase density in neighborhoods.

In other business, the council moved to continue the city's free exterior-paint program, whereby families earning $24,00 a year or less received free paint from the city to spruce up their homes.Since the program began last year, 181 families have participated. Eligible residents may sign up for free paint on April 11 at City Hall.