SAN FRANCISCO — People desperate enough to own a home in this hyperactive real estate market have long resorted to a risky exercise in group dynamics: sharing a multimillion-dollar mortgage on a small apartment building with a bunch of friends or even strangers.

The advantages of the increasingly popular route to homeownership known as "tenancies-in-common" are obvious in a city where three-bedroom homes average about $800,000. The hazards are also well-known and include deadbeat partners and the difficulty of extracting equity.

That's why a bank's offer of individual mortgages to TIC buyers sent shivers of excitement and fear through the city this past summer. By making TICs less codependent and more like individually owned condominiums, the Bank of Marin's so-called fractionalized mortgages were designed to make the option attractive to even more first-time buyers.

But the move, expected to be duplicated soon by other lenders, also has exacerbated suspicions among the city's powerful tenant's lobby that the TIC tool is being used by developers to dump rent-controlled properties and get around San Francisco's strict condominium conversion limits.

Unlike condos, TIC partners do not own their apartments, but only a portion of the building, which leaves the city unable to regulate them as it does condo conversions. In some ways they resemble New York City's co-ops, but smaller and with a decidedly "do-it-yourself" bent.

The San Francisco Tenant's Union, warning that mass evictions would result if the main disadvantage of TIC ownership were removed, is picketing TIC open houses and lobbying the city supervisors to make it even harder for TIC buildings to be turned into condominiums.

"As we turn rent-controlled apartments to TICs which are getting close to $700,000 for a two-bedroom, we are displacing low- and moderate-income people," said Ted Gullicksen, of the tenant union.

While TICs are common in commercial real estate, residential TICs have been a California phenomenon mostly limited to San Francisco, where there are 2 1/2 times as many renters as homeowners and the median price of all homes — single-family, condos and TICs included — was $721,000 in September.

In keeping with strong political support for rent control, the city allows 200 apartments a year to be converted to condominiums through a lottery and those can only be in buildings with six or fewer units.

The regulations were a turnoff for people who went in on TIC buildings — in many cases relying on mixers and online matchmaking services to find partners — hoping they would one day be able to convert their units.

Ann Bassi, a mortgage broker with GT Financial in San Francisco, said residential TICs could spread outside California if state regulators permit them. Cities with expensive housing and condo conversion limits are particularly ripe.

Bank of Marin's new product sparked a mild frenzy despite carrying a higher, variable interest rate and requiring a down payment of 25 percent; its initial $20 million investment was spoken for the minute it was unveiled, pledged to developers looking to transform rental properties.

Randy Brasche, president of a TIC owners advocacy group, predicted that lenders won't find a shortage of customers.

Many people, Brasche included, have been willing to share a mortgage with others they met through a real estate agent because it typically shaves 15 percent to 25 percent off the price of a condo.

But with the savings come potential headaches, such as having to cover for delinquent partners or trying to wrestle equity from the property if someone moves or wants to refinance for the cash.

The notion of buying into a TIC made Chris Freeman, 31, nervous. Freeman, a product manager at biotech company Genentech, paid $1,145 a month for a rent-controlled, two-bedroom apartment on Nob Hill for three years. He went to a TIC open house in his neighborhood last month to check out prices and ended up owning a three-bedroom apartment with his fiancee.

The five-unit, Edwardian-style complex built in 1909 turned out to be the first residential building that Bank of Marin financed with individual loans. Freeman paid $715,000 and his monthly mortgage payments will be $3,500.

The amount was calculated the way it would have been with a group loan, based on the square footage of his unit and a share of the building's property taxes, utilities and other expenses.

"I was not interested in getting involved in a traditional TIC — the lack of control you have, the fact you are responsible for X number of strangers," Freeman said. "We'll have to cancel a few wine club collections, but we'll be OK."

Tenancies-in-common have started cropping up in cities where housing prices are high and growth is limited, such as Santa Monica and Laguna Beach in Southern California, said Andrew Sirkin, a lawyer specializing in TIC real estate. But with an estimated 6,000 to 8,000 TIC units in about 3,000 apartment buildings, San Francisco has the most.

Bill Del Monico, president of Integrated Mortgage Corp., said fears of eviction-happy developers are overblown and the popularity of fractionalized mortgages are exaggerated.

Del Monico thinks many TIC buyers will stick with cheaper group mortgages, despite their complications.