By 1985, the only homes builders will find it practical to put up in Montgomery County may cost $250,000.

Moderate income families who choose to settle in the county may have to settle for mobile homes if they can get in at all.

By then, according to projections, the average resale home may cost $100,000, $30,000 more than the current average in that country.

The county will be increasingly populated by wealthier and older people: so much so that about 20 per cent of the schools currently operated in the county will have to be closed.

This is the future as outlined yesterday by builders, planners, natural resources experts and county official sat a forum sponsored by the Democratic Party.

Montgomery's future will be dictated to a large extent by decision county officials have already made. The seven-year sewer moratorium, in addition to preventing the housing supply from increasing, has simply created a situation "in which there is no cheap land in Montgomery County" according to Michael Sumichrast, vice president of the National Association of Homebuilders, who was part of a panel on housing.

Housing in the county reflect the "disproportionate" land values in the county, according to builder Martin Poretsky, president of the Suburban Maryland Homebuilders Association. "If a builder is paying $50,000 for a new lot (lot with a sewage hook-up), he's only going to want to put up a $250,000 home on that lot," Poretsky said.

Members of the housing panel also saw a bleak future for apartment availability in the county. The county's current stance against high density zoning, they said, will discourage private investors from putting up apartment houses.

Those investers who do decide to build will be putting up only luxury apartments, Sumichrast said. Any other new construction of apartments will probably be financed by the federal government and serve the needs of the very poor or those on fixed incomes, the panelists projected.

"There will be nothing for those in between," Sumichrast said.

The county is already one of the nation's wealthiest and costliest and most exclusive. Its 600,000 residents now have an average income of $26,000 per household. According to previous projections, average household income will exceed $45,000 by the early 1980s.

Although it was projected that more blacks (beyond the current 9 per cent) will be moving into the county in the next eight years, these will be high-income families that come from the District of Columbia.

It is estimated that one-third of the county's population will be over 45. As the average age of county residents increases, there will be fewer children in the county requiring even more public schools to close.

Transportation officials disagreed at yesterday's forum over what environmental and economic impact the extension of the subway system into Montgomery County will have.

William Herman said the extension of the system will be "a positive tool" in revitalizing certain areas in Montgomery County where Metro stations are to be located.

He estimated that in the next five years, 50 percent of the people who commute to the District from Montgomery will be using public transportation. Herman also said that by 1985, the per passenger cost of running the subway will actually be cheaper than per capita cost of providing bus transportation.

Gerald Miller, a transportation analyst for the Urban Institute, estimated that only one out of every eight commuters can be expected to use public transportation which raise the question whether the other seven should be financing the travel of one person," Miller said.