Renters in Washington and its suburbs will have to search even harder to find housing in future years, as apartments become costlier, more unkempt and harder to find, according to a report released yesterday by the Greater Washington Research Center.

The report drew a gloomy portrait of city and suburban tenants already living in deteriorating apartments, paying more than one-third of their incomes for rent and still facing the threat of displacement by condominium conversions.

The rental housing market is already crowded, but few new apartment buildings are being constructed, the report says. Although many apartments are deteriorating, landlords would rather abandon the properties than repair them or continue providing services in the face of double-digit inflation.

The "sickness of rental housing" is creeping over Washington and its suburbs, the reports says, and it is not likely to be cured any time soon.

"For renter households, these trends spell fewer choices, reduced services, and rising rents in the years immediately ahead," the report said.

The author of the report, Morton J. Schussheim, said the most severe problems appear to be in the District and Prince George's County, both of which have large concentrations of poor people and many older apartment buildings that are not being maintained adequately.

Schussheim said the problems in the Washington region are not as severe as in some other large cities such as New York, where massive numbers of landlords already have abandoned their buildings.

But this area could suffer extensively from "walk-aways" within the next five to 10 years unless local governments provide incentives to encourage owners to stay in the rental business, he said.

While other cities may be worse off than the Washington area, there are many problems here crying out for attention that the report called "serious and growing."

Construction of apartments in the Washington region is likely to dwindle to a "trickle" in future years. Already, construction has dropped from more tha 15,000 apartments each year in the early 1970s to fewer than 6,000 annually in the late 1970s. Nearly all recent developments are subsidized housing units.

About 11,000 renting families in the area pay an uncomfortably high 35 percent of their income or more for shelter. Of those families, 70,000 live in the suburbs.

Tens of thousands of apartments in the suburbs and more than 60,000 in the District suffer to some degree from defferred maintenance. Some property managers say they curtail services because rents lag behind increases in operating expenses.

The renting population increasingly consists of the "have-nots' -- the young singles and couples without downpayment money or steady incomes, low-income families, minority families, the elderly and others on fixed incomes.

Such residents generally demand more financial assistance and more stringent rent controls, and that could forecast more tenant-landlord conflicts in the future, the report said.

The report lent support to the argument that upper-income tenants gain the most benefits from rent contol laws, not the poor.

"People who are newcomers are at a disadvantage with long-term renters," Schussheim said. "After a period, rent control confers a bargain upon staying in place." Newcomers may be forced to share houses or rooms, the report said.

In the area of condominium conversions, Schussheim's report said a policy of permitting conversions to proceed in an "orderly" way seems better than a "wholesale and extended freeze on all such activity."

He said both Fairfax and Montgomery counties have what he considers to be "orderly" approaches. The District has imposed a temporary moratorium on most condominium conversions while the City Council considers new permanent legislation.

Schussheim, a senior specialist in housing at the Congressional research Service of the Library of Congress and adjunct professor of planning at Howard University, said he is "gun-shy" about pointing to a single answer to the area's problems.

But he said that several approaches, many under consideration by local officials, are noteworthy.

Schussheim suggested that housing assistance payments be made to lower-income tenants, probably from federal funds.Such a program would require $25 million in the District annually to provide $500 a year for an estimated 50,000 families, he said.

He also suggested moving moderate-income tenants into ownership housing, such as low-yield cooperatives that ensure buildings always house lower-income families.

Local officials could make better use of federal laws that provide low-interest loans and refinancing money to fix up deteriorating buildings, he said. Programs that absorb rising utility costs at apartment buildings could be instituted, and he praised Montgomery County for considering changing building code requirements to encourage apartment construction.

"One of the most effective actions local goverments can take to improve the investment outlook is to maintain municipal services in moderate- and low-income rental neighborhoods," the report says.