Home prices and rents will not decline nationally during the coming decade, contrary to some predictions of a housing market crash, but renters will continue having trouble finding the money to buy their first homes, according to a new Harvard University housing study.

The good news for current owners is bad news for young, first-time buyers waiting for prices to come down to a level they can afford. Only 20 percent of white renters and 4 percent of black renters will be able to buy homes in the coming years, the researchers concluded. Harvard's Joint Center for Housing Studies issues an annual survey of the nation's housing and focused much of this year's report on the outlook for the next 10 years.

Builders will erect fewer houses than in the past decade, the report said, but "construction activity will still increase in the decade ahead" because the homes will be bigger and better structures will be designed for affluent move-up buyers, the survey said. The researchers also predicted that some owners will renovate and repair their homes instead of moving, boosting this segment of the construction industry.

Rent levels throughout most of the country have flattened "because of the economic slowdown and record vacancies in some overbuilt markets," according to the study. Nationally, the median rent has stabilized at $409 monthly, just $2 below its peak in 1987. The median is the point at which half of all apartments rent for more and half for less.

Home and apartment construction will likely remain at current levels for the next few years and then drop in the mid-1990s, a reflection of the nation's changing demographics. More than half of the baby-boom families already are homeowners and often are expanding existing homes rather than moving to different ones.

Meanwhile, there is growing poverty among renters caused by a mix of economic, social and political factors, according to the study. Income from government assistance programs has "lost ground to inflation" while higher divorce and teenage pregnancy rates have increased the number of poor, single-parent households.

"The outlook for low-income owners and for renters who never make the transition to homeownership is far less optimistic" than that of more affluent Americans, the survey said. More members of these groups will have to pay "large shares of their income for marginally adequate housing," the researchers said.

As low-cost rental units have been "lost to abandonment or upgraded to attract higher-income occupants," the number of poverty-level households paying more than half their income for rent has reached about 6.1 million, according to the study. Although they pay a high share of their incomes for rent, "more than a third of these households live in structurally inadequate units," according to the researchers.

William C. Apgar Jr., acting director of the Harvard center and an author of the report, said the slowing of the residential construction industry creates "an opportunity to redirect national resources to address the unmet needs of low- and moderate-income households."

The study, due for release today, was funded by the Ford Foundation and associations representing the housing and housing finance industries, state governments and low-income renters and homeowners.