The Montgomery County Planning Board, for the first time, has approved plans for a mobile home development, explaining that such dwellings could offer a new approach to the problem of building lower-cost housing in one of the nation's wealthiest suburbs.

Approval for the 366 mobile units, to be located on a 100-acre site in Germantown, came on a unanimous vote yesterday. As Commissioner Mabel Granke put it, Montgomery County housing in the $31,000 to $35,000 price range was "due yesterday."

However, some residents near the property at the intersection of Frederick Road and Rte. 118, and some of the builders constructing other developments near 1-270 in Germantown complained yesterday that their area is receiving a disproportionate share of the lower-priced homes in the county.

In a dramatic two-hour presentation, complete with a coterie of lawyers, consultants and mulicolored maps, builder Maurice H. Berk successfully persuaded the planning board to agree to his project immediately, rather than adhere to a long standing Germantown master plan that calls for development of his land several years in the future.

Even mobile home costs are rising out of the reach of the people most likely to want them - the elderly on fixed incomes and younger families buying their first home, Berk contended.

"Don't make the mistake of waiting and waiting, thinking mobile homes will always be available (for moderate-income housing needs) because they are going up in price too," Berk's lawyer, Stephen Kaufman, told the board.

The problem of allevieting the shortage of moderately priced housing in Montgomery County - whose median family income of $27,000 is the highest in the nation - is exacerbated by scoaring land values and the constant demand for higher-priced housing by the County's current and new residents.

Vacant lots cost as much as $60,000 in the wealthist sections of Bethesda and Potomac, and the average single-family home in the county is now priced at $70,000. Yet county projections show that about 4,700 new low and moderate-income units will be needed each year for the next decade.

While other jurisdictions have in some cases reluctantly turned to mobile homes to fulfill lower-cost housing demands, there has never been much of a push for such units from Montgomery residents.

In fact, when the county's first formal ordinance allowing mobile home development was adopted in 1974, some supporters of mobile housing complained its controls were so strict that no mobile home development would be built.

Since then, only Berk has applied to build one.

"This is an opportunity for residents of this county to see what a modern mobile home development looks like," testified Peg McGrory, a housing consultant in urging passage of Berk's plan.

At present, most Montgomery residents' image of mobile housing is based on the county's five conventional trailer parks whose 355 units are stacked side by side perpendicular to Frederick Road in Germantown. Another 250 mobile homes are illegally scattered around the county.

The planning commission also considered the illegal mobile unit problem yesterday by approving a zoning amendment that would require these homes to comply with health standards in order to remain where they are.

Both planning commission actions will go to the County Council for final consideration.

The Germantown Citizens Association opposed Berk's project only on the grounds that the revised timing of the development would disrupt the orderly growth of their community, said Helena Kuklewicz.