One morning last year, tenants of the Townhouse Square Apartments in Dumfries awoke to find a notice stuffed in their mailboxes stating that their rent would be raised from an average of $300 a month to $50 a day--an increase that amounted to about 500 percent.

It was to be a historic moment of sorts for Prince William County--but right then no one was thinking about that.

"The owners clearly were clearing them out to pave the way for conversion to condominiums," said county legal aide lawyer Amy Oppenheimer. It so angered the tenants that they decided to fight the rise.

"That was the first case I can remember of contested condo conversion" in Prince William County, said Oppenheimer last week. "I doubt it will be the last."

Conversion to condominiums, once thought to be solely an urban phenomenon, has crept into the suburbs. And officials in Prince William, 30 miles west and south of Washington, say it has now crept into their still largely rural county as well.

"We are like the frontier for development in the District of Columbia area," said Hubert King, an investigator in the county's Office of Consumer Affairs. "If you see something in Montgomery or Fairfax counties , the chances are you'll see it in Prince William next."

The reasons behind the conversions are the classic ones of a rapid population increase coupled with expensive and scarce housing. For lower- and many middle-income people interested in home ownership, the only financially feasible route is often to buy a unit in a condominium, county officials said.

In the Townhouse Square case, an agreement was reached between the complex's owner and the county that has allowed the complex to be converted in phases without undue disruption to the tenants or the $50-a-day rent, said Oppenheimer. But in response to that hotly debated conversion, Prince William has begun a media campaign to alert thousands of other county apartment dwellers of their rights as tenants.

"There is no rent control in the county, but people have a ledge to stand on when it comes to conversion," said Deborah Koss, county consumer affairs director. State law provides some relief by mandating that apartment owners give tenants 120 days' notice of intention to convert. Tenants also have the right of first refusal to buy their units.

So far, three apartment complexes in Prince William have converted to condominiums, said county planner Keith Palmatier. The first to go was the Prince Cole complex in Manassas in 1976. That was followed by the Moorings of Occoquan in 1981 and Townhouse Square this year.

He said there are five other residential condominiums in the county that were built as condominiums plus approximately 50 other apartment buildings, none of which currently faces conversion.

County officials said they expect to see an increase in condominium conversions and applications as more and more people move into Prince William, one of the fastest growing counties in the nation, according to recent census figures.

"There will be other condo conversions and we want it to go smoothly," said Koss. "County services are becoming more familiar with condo law."