Builders around the world must redirect their efforts to smaller, multifamily dwellings if they are to cope with dwindling housing resources and a mushrooming population, a new study for the United Nations concludes.

Americans especially will have to adjust to smaller homes or be prepared to pay as much as $150,000 for an average house in nine years, said the Worldwatch Institute, a Washington-based reasearch organization.

"Trends in each of the major housing resources all point toward the building of the same kind of houses -- low-rise, multifamily houses and town houses," said Bruce Stokes, a Worldwatch researcher who wrote the report.

A projected 35 percent jump in the world's population by the year 2000 would push the census to about 6.1 billion and would add an estimated 600,000 households to the already sparse housing market of the lesser-developed world, the study concludes.

The United States will face a wave of new house-hunters as the 1950's "baby boom" generation begins settling down and as changing social patterns produce an increasing number of one-person households, Stokes predicted.

U.S. home construction costs tripled in the past decade, and material costs outstripped consumer price index increases by more than two percentage points during the same period. Since 1949, Stokes said, land and financing costs have nearly doubled their proportions of the total new-house bill.

"We have to accommodate to a world of unrenewable resources -- and accommodate ourselves by down-sizing our houses, changing the types of materials we use to build our homes and changing our energy consumption patterns," Stokes said.

Stokes predicted that the present U.S. home will become the "dinosaur" of the 1980s, much like the full-sized auto became outdated in the '70s.

"The single-family dwelling as we know it in the United States is a quirk of history, of economics," he contends. "We had a lot of cheap land, cheap money and cheap and plentiful materials."

By the end of this decade, Stokes predicted, the typical American house may shrink to less than 1,400 square feet from the 1978 high of 1,527 square feet.

Although the average size of houses has declined slowly in North America in the past three years, the typical American home is nearly triple the average size of a new unit in the Soviet Union and one-third larger than West German homes.

The Third World, long-plagued by inadequate-housing problems, will have half again as many people looking for shelter at the turn of the century, the report predicts.

Noting that most Third World families build their own homes with whatever material is available, Stokes said the crucial issue there will be how to improve building techniques.

Houses of the future should be clustered to avoid land waste, the Worldwatch report recommends. As an example of such grouping, Stokes points to Village Homes, a 200-household development in Davis, Calif. The builder clustered eight or 10 houses on one section of a land tract and left a large "backyard" where the community cooperatively maintains vegetable gardens, fruit trees and a playground. That keeps 12 of the development's 77 acres in agricultural production.

Stokes said architects must redesign home interiors to use space more efficiently, perhaps by eliminating such luxuries as dining rooms or by creating multi-use rooms like the Japanese -- who often use sliding panels to transform sleeping rooms to dining or recreation areas.

Citing an estimate by William C. Baer of the University of Southern California that more than half of America's housing units have surplus space, he estimated 1.7 million new housing units could be created by making too-large houses into duplexes.

Reducing world housing demand in the next century will cost governments $3 billion each year, Stokes estimated, He suggests leaders of industrial countries give tax incentives for high-density households, remove unnecessary construction regulations and oversee the development of new plastics and earth-based building materials.

The U.N. Fund for Population Activities funded the study, which Stokes said cost the fund approximately $40,000.