* The median price of a new house in the United States will hit $150,000 by 1990, about double what it is today, even as it's tripled in price the last 10 years.
* Housing conditions in the developing nations of the world will steadily worsen because of a sharply rising demand, plus continuing constraints on the use of natural resources.
These are the grim conclusions of a new study by the Worldwatch Institute, a Washington-based think tank.
''Just as during the 1970s, when energy constraints redefined the size and use of the automobile, housing resource constraints in the '80s will reshape the average new home and change the living patterns of people everywhere,'' the study declares.
The outlook for third world nations is especially bleak, according to the study.
''In the developing countries,'' the report by Bruce Stokes predicts, ''there is little likelihood that most people will ever live in homes of the standard now found in industrial countries. The Western housing model will frequently be abandoned in favor of traditional building techniques that are more in tune with available resources and people's incomes.''
Housing production, according to a United Nations estimate, is running up to 5 million units behind demand in the third world, with hundreds of millions of people living in unfit conditions today.
The study, entitled ''Global Housing Prospects: The Resource Constraints,'' estimates that ''1.8 billion people do not have access to adequate sewage disposal and 1.3 billion lack clean water.'' The situation is exacerbated in such countries as Kenya and Nigeria, where the population is expected to double in the next 20 to 25 years, with the number of people in the household-forming age group rising by 600 million, nearly twice the number looking for homes today.
In Egypt the urban housing shortfall in 1975 was estimated at 1.5 million units a year; in India in 1973, it was put at 3.8 million units.
Even such high-technology nations as the US face an uncertain future in housing because of ballooning costs.
''The single-family, free-standing house is a peculiar development based on a unique combination of cheap capital, energy, land, and materials,'' asserts Mr. Stokes, a senior researcher with the institute.
''The home of the future will be built with an eye toward economizing resources. More people will live in town houses or apartments that are smaller and have fewer conveniences than people have come to expect.''
In fact, the average size of a new home in the US fell from 1,527 square feet in 1978 to 1,464 square feet in 1980, ''the first time,'' says the study, ''that a traditional measure of housing quality has reversed direction.'' The size of building lots is also getting smaller.
In the developing world, however, up to two-thirds of the people cannot afford to buy even the cheapest conventional dwelling.
To alleviate the crunch, Stokes says that governments need to step up their efforts for population control and encourage patterns of household formation - house-sharing, home care of the elderly, and the creation of more rental units - that conserve housing resources.
The study, which looks at shelter prospects on a global scale, was funded by the United Nations Fund for Population Activities.