In the turbulent 1960s, communal living was something left to the social dropouts of the counter-culture while mainstream Americans continued to develop the nation's ubiquitous housing tracts. Today, things are changing fast.

In California, grossly inflated housing prices have made the affordable single-family home as scarce as a button-down collar in Haight-Ashbury. The thought of two families living in the same house and sharing the mortgage is an idea whose time has come.

From San Francisco to San Diego, more and more families are sharing single-family houses. Housing experts say the trend probably will spread to the rest of the nation as 20 percent mortgages and escalating home prices drive buyers out of the market.

"The dream of a young married couple buying their own home is fading fast," said Thad Walker, an Orange County real estate broker. "It is not uncommon now for two or three different people to team up and buy a house -- divorced mothers, elderly persons, young couples. It's the only way 60 or 70 percent of the people can afford to buy."

With new homes in California averaging $120,000 ($150,000 in Orange County), several builders are discovering that they can no longer buck the trend. At least one already has broken ground on what he says is the nation's first home expecially designed to be shared by more than one family.

"This could very well be the house of the future," said Randall Lewis, director of marketing for Lewis Homes Inc. He is building the unique home in concert with the Asphalt Roofing Manufacturers Assn. "It's a matter of pure economics," he said. "Two can buy cheaper than one."

Lewis Homes, the nation's 19th largest homebuilder, has real estate agents and other builders closely watching the project in Upland, a suburb about 40 miles east of Los Angeles.

"If this thing goes over, we may see the end of single-family homes, except for the wealthy," Walker said. "Communal living or home sharing or whatever you choose to call it, is just around the corner."

Lewis and Santa Barbara architect Barry A. Berkus have designed a house they say combines privacy with efficient common areas, such as kitchens and living rooms.

"Each family has its own bedrooms, bathrooms, and dressing rooms," Lewis said. "But both families share the living room and kitchen, both of which are oversize."

In addition to the living room and kitchen, the two families would share a Jacuzzi in the center of the home and a swimming pool wrapped around two sides of the 3,000-square-foot house.

Other features in the home Lewis calls the "Style Setter 80s Home" will include a 375-square-foot lanai, a living room which is detached from the house and which can be used as a master suite for a third family. Inside the house, Berkus has designed lofts, skylights and plenty of open space.

"The interior is one grand room with areas separated by levels rather than a series of rooms," Berkus said. "The opend floor plan adds volume and spaciousness to the interior and makes the home a refreshing departure from formal design."

The total shared space inside the house is 1,540 square feet -- the size of a large condominium or moderate one-family home.

The house will have two private master suites containing a bedroom, bath and dressing room. Each suite is 397 square feet.

Another 650 square feet is taken up by the indoor-outdoor patio, closets, storage areas and lanai.

The home also will include a solar-energy system. Cavities in a loft that will rise in the center of the house will function as a return air-system, pulling warm air from the skylights and higher lofts and distributing it through a floor-level duct system.

"The house is built to take advantage of the sun's warmth in the morning and to deflect its heat in the afternoon," Lewis said. "This is a total energy-efficient home."

A sunken entertainment area will include a built-in television set with a video tape recorder and a giant screen.

"We are building this house to accommodate what we see is a changing way of life in America," Lewis said. "More and more families are spending weekends and vacations at home because of higher costs of automobile and airplane travel."

The house, which is scheduled for completion next summer, also will offer fire-retardant asphalt shingles, a definite plus in an area that only recently battled a rash of devastating brush fires.

What will it sell for? About $300,000. "We expect this prototype model to be purchased by young professionals," he said.

He said other homes to be built later will be designed more for families with children, and the asking prices will be lower.

Lewis conceded that some oppose the idea of the house. "A few of the neighbors are afraid we're putting up some kind of tenement," he said.

Zoning ordinances in most communities are set up to protect the single-family home. Now, with home sharing around the corner, many communities are rewriting their laws. In one recent zoning challenge, the California Supreme Court ruled that communities cannot restrict the number of unrelated adults living in one house.

"All we're doing is moving the ghetto into the suburbs," one homeowner said. "What else would you call it when you can have three or four families crammed into one house?"

In spite of such opposition, classified sections are filled with ads offering converted garages as "master suites" and homes having "in-law apartment potential."

The "in-law" phrase often is used by real estate agents who realize that few in-laws will live in the apartments. This is done because in-laws are technically family members, and having separate apartments for them does not violate local zoning laws.