Rising home prices and a declining number of housing units in the Washington area are pushing low- and moderate-income residents into counties farther beyond the metropolitan area than before or into homeless shelters, local housing officials said this week.

A shortage of affordable housing "is a new problem for our county," said Prince William County Supervisor William J. Becker. Many local government leaders in the metropolitan area "were elected saying we would do something about housing problems, but we don't know what to do."

Once a haven for people seeking a lower cost of living, Prince William has joined the ranks of counties closer to the District, like Fairfax and Montgomery, that have struggled to provide affordable housing for years. Rent for a two-bedroom apartment in Prince William now ranges from $500 to $700 a month and, while most houses still sell for between $90,000 and $200,000, some new luxury developments today contain $500,000 houses.

Becker and about 140 other representatives of local governments, nonprofit organizations and private industry attended a series of meetings on housing problems sponsored by the Metropolitan Washington Council of Governments (COG).

In the wake of a 70 percent cut in federal funds for housing since 1980, local housing authorities must look for new sources of funding. Bernard L. Tetreault, executive director of the Montgomery County Housing Opportunities Commission, said agencies will have to save themselves because "the feds are not sending out any life preservers."

Local governments and nonprofit organizations will not be able to match the amounts once provided by the federal Department of Housing and Urban Development, several speakers said. As a result, local authorities must look for new sources of funding from private companies, according to Tetreault.

A 1985 COG study showed that more than 134,000 households in the area need housing assistance but are not getting it, the organization said in a report distributed at the meetings. Since then, housing prices have soared while condominium conversions and renovations "have removed thousands of low- and moderate-income housing units from the housing inventory," according to the report.

Thousands more low- and moderate-income residents are expected to be displaced when current plans for renovating rental projects in Arlington and Alexandria are carried out, the COG said, adding that the uprooted residents "have little hope of finding similar affordable housing elsewhere."

A few years ago, these tenants might have moved to the outlying Washington area counties, but those days are over.

In Prince William County, the population and number of housing units have quadrupled since 1960 and people keep moving in, said John Schofield, director of the county office of planning and zoning. "Increasingly, we are seeing people moving further south, to Stafford and Spotsylvania counties." Rents in most houses and apartments have risen beyond the reach of low- and moderate-income residents and the number of units once available to poor families is dwindling, he said.

Prince William's population growth rate has risen from 3 percent to 4 percent a year at the beginning of the decade to 5 percent to 6 percent now, while the number of elderly residents will have doubled by 1990 to 8 percent of the total population, according to Schofield. The county is beginning to experience the problems of urban jurisdictions, such as "more police calls," an increasing number of homeless people and an increasing percentage of renters compared with homeowners, he said.

Families moving to Prince William and other outlying counties also must take into account the cost of commuting to jobs in the District and elsewhere in the metropolitan area. People moving into Prince William will face about $300 a month in transportation costs, not enough to make up for any savings in rent, said another county official.

To the north of Washington, Frederick County will soon have "urban sprawl" and other problems characteristic of cities, said J. Anita Stup, president of the county board of commissioners. Housing costs are rising, and while many houses are priced at less than $200,000, some large houses in the southern and eastern parts of the county have price tags of up to $500,000, she said.

"Frederick is a very conservative county. The commissioners have not put any money into affordable housing" for low-income residents, Stup said.

Local governments in neighboring Northern Virginia face a similar situation, several speakers said.

"We started at square one in the state of Virginia. The General Assembly has not focused on housing at all," said Arlington County Board member Albert C. Eisenberg. With a 2 percent vacancy rate in rental housing, the county has difficulty finding homes even for low-income citizens who have federal rental subsidies that pay 30 percent or more of their rents, he said.

Housing is beginning to get some attention in Richmond, however. Gov. Gerald L. Baliles last month asked the General Assembly for a $45 million housing fund to preserve and expand the state's low-income housing stock. Earlier this month, a state Senate subcommittee proposed an alternative to Baliles' plan. The subcommittee would like to establish a revolving loan fund for low-income housing to be administered by the Virginia Housing Development Authority.

The Northern Virginia Housing Coalition, made up of local government officials, nonprofit organizations and representatives of private industry, has proposed a housing fund to which the General Assembly would allocate $100 million annually to help local jurisdictions and private developers build low- and moderate-income housing.

The price of land is so high in Fairfax County that low-cost housing cannot be built, said County Supervisor Thomas M. Davis III. "We've got to bring the cost of land down."

The need for low- and moderate-income housing, however, conflicts with the policies of a "slow-growth" county board that has pledged to hold down development, Davis said. The most frequently used inducement local governments offer private developers to build low-income housing is permission for the builders to put more market-rate housing on their properties than zoning regulations normally would allow.

Fairfax County residents feel the county's most pressing need is improvement of the transportation system, closely followed by more money for the school system, Davis said. "Housing is not a high priority, although more citizens are conscious of the need" than in the past.