The American dream house of the future not only will be smaller, but may have to be shared.

Communal living has already arrived in the housing-hungry West and may signal the beginning of a new living style unless the crisis in construction eases, according to experts at the recent Pacific Coast Builder's conference here.

"People will have to learn to accept smaller units and have density in housing," predicted George Gentry, president of the builders' group, who admitted he had been "shocked" when he discovered that three families were living in some new three-bedroom units and even doubling up in one-bedroom homes.

Robert H. Rivinius, an executive vice president at the conference, forecast a growth in communal living, with people "buying an interest in a common area of the living room and kitchen." That trend already is being demonstrated, he said, in three- or even six-bedroom units that were built around a large community living area.

"We may be approaching the era of the multifamily unit. Housing in this country is going to change drastically," Rivinius warned.

The builders stressed that they are not anticipating the disappearance of the traditionally concept of a "family home on its own ground, surrounded by a white picket fence."

But "that is likely to cease to be the typical house as we have known it," Gentry said.

Merrill Butler, president of the National Association of Home Builders, said the key consideration of the decade is simple: "We have to start thinking about where our children will live. Housing and building policies established now will have great impact on their living standards."

Officials and delegates at the three-day convention admitted part of their pessimism about the future of housing in the United States is based on the current depressing statistics. Housing production nationally is down more than 50 percent from last year, and about 1 million people are unemployed in the building industry.

Yet there was acknowledgment that the housing recession should bottom out in the third or forth quarter of this year and that sales gradually will rise as interest rates drop.

But some warned that the "pent-up demand" by prospective home owners unable to buy because of high mortgage interest rates could unleash an even more "virulent bout of inflation" in housing prices when credit eases."

That possibility is one reason builders have begun to focus on "affordable housing," which some describes as "the compact house."

According to Sanford Goodkin, a housing industry analyst, "The next big change in houses is sizing -- and that means down."

He predicted there will be an increase in factory homes, which are built in components and then set up at the home site instead of being put together part by part. Such homes, he said, would less labor and help bring down costs.

The day is not far off when homeowners will consider 1,000 square feet of floor space large and will be accustomed to living in 400 square feet, Goodkin said. Today's detached homes average 2,300 square feet. By the mid-1980s, the analyst said, it is likely that the average size of new housing will be half that figure and will come in attached, rather than detached form.

W. Scott Biddle, president of a southern California development company, said the transition is in its infancy, but he suggested that in the "foreseeable future," tracts of suburban family detached houses will be replaced by town houses, condominiums and highrises.

The impact is likely to be felt first in California, perpetually a national trend setter, according to housing officials. With the highest and scarcest housing in the country, Californians probably will be the first to see a change in their home life styles -- the disappearance of the large fenced backyard, the family pool, the extra bedroom or the family room and, most shocking of all, the two- or three-car garage.

It was forecast that the condominium boom of the current decade eventually will transform suburbs into urban neighborhoods, with the traditional home of the middle class becoming something financially practical only for the wealthy.