Challenges, choices for home buyer,

Shared housing, which carries a reputation somewhere between a commune and a college dorm, has probably never been thought of as profitable. But across the Golden Gate Bridge in Marin County they have a different perspective - especially when it comes to housing.

Here, where steep real estate prices coexist with ''alternative life styles, '' where even a 100-year-old Sausalito ferryboat is subdivided and rented out, capitalism and kibbutz are not mutually exclusive concepts.

In the next year, 12 middle-income adults who may not know each other today will become co-owners and co-habitants of three $350,000 Mill Valley homes designed for communal living. They'll pay $17,000 down and $555 a month for their seven-bedroom, four-story houses to be built on Mt. Tamalpais, overlooking San Francisco Bay. Assuming they rent out two rooms in each house, the monthly cost after tax write-offs will be $275 a month.

The homes are designed to ensure the privacy of residents while also allowing them a family-like atmosphere if they choose it.

''This is a demonstration project to show that home ownership is affordable when shared ... that you can afford high-quality housing (in a group setting) ... and to prove this can work in the marketplace,'' says Ann Howell, executive director and founder of Innovative Housing, the nonprofit group putting the Mill Valley project together.

Ms. Howell's idea came when she was researching Israeli kibbutz life as a Chicago journalist 15 years ago. In the meantime she earned her development credentials in masterminding San Francisco's Fort Mason, a showpiece of urban renewal using abandoned government buildings on waterfront property.

Shared housing is not new, she admits, but the notion of marketing it as a mainstream life style is. And Innovative Housing has enlisted influential backers, including environmentalists, architects, lenders, housing officials, community leaders, and academics.

Maintaining the aesthetics and profitability of the large home, but building it to accommodate more people, also helps bridge the gap between the environmental and development communities, says Greg Archbald, founder of the Trust for Public Land and a supporter of the new project.

The housing crunch has given rise to co-ownership and smaller home designs - like 600-square-foot minicondos - as the only way for many to realize the American dream of home ownership. The older nuclear-family style house and the new smaller homes don't fill the needs of a growing demographic bulge - single parents, divorcees, the elderly.

Innovative Housing tries to legitimize shared households by providing a support system to make it profitable financially and socially. It finds the financing, researches real estate and tax laws for this kind of ownership, provides workshops for owners to find compatible housemates, provides mediation service, acts as an agent for those who want to rent out extra rooms in the house, and oversees resales, preventing speculation by setting prices according to consumer price indexes.

On a $17,000 grant from the San Francisco Foundation, Innovative Housing leased 23 Marin County homes last year to test concepts for the ownership program. The rental project showed that there was a demand for group living and proved to reluctant lenders that households of unrelated people can be stable, says Ms. Howell. The program placed 120 residents (44 percent single parents and children, with median age in the mid-40s) in spacious homes. Rents vary from $ 200 to $500 per person. And Innovative Housing generates its operating income from monthly resident fees. The group is also developing two projects for lower- to moderate-income groups.

The projects indulge the American dream by offering spacious living areas, affordability, and an old-fashioned extended-family support system.

''People tend to spend time like this (shared housing) in college and they look back at that as the best years of their life because they were less alone, less alienated, less separated from the world,'' explains Richard Bender, dean of the environmental design school at the University of California, Berkeley, and a member of Innovative Housing's board of directors.

Design students find they have to capture the essence of that group situation in homes, because a house ''can't be defined by a mother, a father, 2.5 children , and two cars. In the real world ... there is a much wider range of family structures with a much wider range of space types, and variations are in demand, '' Dean Bender says. In the Bay Area, for example a majority lives in ''nonfamily households,'' according a study by the Greater Washington Research Center.

The image of shared housing is often negative, but that's because it hasn't been adequately promoted, says Bender. ''These things may appear to be strange, but a generation ago the thought of living in a minicondo would have been shocking'' to those whose values centered on the notion of extended family, he adds.

One Marin County community rejected a proposed rental project for group living, concerned that it would lower property values. ''There's the idea people wouldn't do this if they could afford something else, and so communities think this would bring poorer people into the neighborhood and pull prices down,'' says Bender.

Richness of family life as well as economy led Linda Purrington, a single mother, to Innovative Housing. Workshops help form compatible groups that choose a ''character'' for the house. Ms. Purrington and one-year-old Cheyenne live in a home with a retired nurse and two single fathers whose toddlers visit regularly.