About a half-mile from California's fastest-growing town, signs sprout from the alfalfa fields, heralding a new sort of growth: The Estates at Dallas Ranch; Highland Ranch at Somersville; Country Living at Toureville; California Grove -- Now Selling; Summerset -- 14 Floor Plans.

The signs portend the future of Brentwood, a town of 18,000 -- for now -- between Stockton and Antioch. "I guess this means Brentwood isn't going to be a small farm town anymore," says Guiga Arno, a thin woman with a winning smile who sells cough drops and soda at Brentwood Boulevard Beacon Gas. "The sad thing is that those fields, the farms, were the best thing about living here."

Down Highway 4 from Arno's gas station counter lie hundreds of acres of freshly bulldozed fields, sprinkled every half-acre with bales of plumbing and electrical conduit, framed with cul-de-sacs to nowhere. Farther south, more such fields host half-finished houses the size of small apartment buildings. At the edge of all this, inside a row of grass-trimmed, newly completed sample homes, Linda Russell "sells dirt," as the practice of flogging unbuilt houses is known. Russell's employer, Morrison Homes, is one of a dozen builders creating subdivisions out of farmland around Brentwood, offering affordable luxury homes like the Hallmark ($239,990), the Cardinal ($256,990), and the Tribute ($266,990).

San Francisco is 45 miles to the southwest, Silicon Valley farther, commutes of at least three hours a day on congested highways. But the buyers keep coming. "I've had 50 people who commute to San Francisco," Russell effuses. "A lot of them commute to Silicon Valley, to San Jose."

Brentwood's newest residents get in their cars each workday and they drive: down Highway 4, through the 10,000-acre housing tracts of Antioch, through the endless office parks of San Ramon, past Hayward and San Leandro and Oakland, and across the Bay Bridge to San Francisco.

Here, on a sunny Sunday in August, an enthusiastic carpenter and sometime real estate salesman named Raymond Morgan bounds across the floorboards of a smallish house on Wisconsin Street, in the once-blue-collar neighborhood of Potrero Hill. Morgan sweeps his arm dramatically to take in the house's small interior, talking of amenities to come. It has potential, he says. It has a great view. The small yard could be a gardener's delight.

Morgan's verve seems odd, given that the house, which is selling for $439,000, has been gutted to the frame. There are no interior walls, plumbing, electrical wiring, bathrooms, doors, or any of the other niceties that make a house a home. It will cost at least another $100,000 to finish the place, and that would be in the humblest way.

Still, Morgan remains untempered. The price is low considering what other houses are going for in the neighborhood. "There was a place down the block that went for $700,000, and it didn't have near the view of this one," he says.

A pack of home-seekers -- three young couples, an older man, two young men -- who have spent the morning plodding through such houses stare blankly at Morgan's bare-stud walls, peer down holes in the plywood where toilets used to be, then walk silently out, expressionless. Like members of the ghost pack that roams the city's available rental apartments -- whose prices have similarly gone up by 60 percent during the past five years -- many of these people will eventually give up on the idea of living in San Francisco.

"A lot of them," Morgan confides, "they're not serious."

While they may seem worlds apart, the lives of Arno, Russell, and Morgan are inexorably linked, and, in turn, deeply intertwined with the destiny of San Francisco. Each of them plays a role in the swiftly evolving San Francisco-area housing market, which during the span of a half-decade has gone from being merely expensive, to one of the most inaccessible in the world. An average two-bedroom/two-bath apartment renting for $1,556 a month in 1994 went for $2,239 last year, according to the latest available figures, and rents continue to go up.

This means couples with children are generally shifted toward the bottom of the pages-long lists of applicants for scarce vacant apartments. It means people of modest or even middling means don't even think about living here. It means that ravenous home-buyers bid for properties, pushing sales prices well beyond the already-exorbitant asking prices.

It means that the future for many who work in San Francisco, or simply want to live in the area, lies in places like the alfalfa fields of Brentwood. Suburbs 45 miles away -- which one might ordinarily imagine as bastions of white flight -- are beginning to appear more economically and racially diverse than San Francisco, as high housing prices push people ever farther away.

The Bay Area urban sprawl, which had been crawling toward the Central Valley for decades, has begun to race toward the Interstate 5 corridor, a minimum one-hour drive from workplaces in San Francisco and on the lower Peninsula. Now, every day during working hours, San Francisco's population of 780,000 increases temporarily by around 215,000 people, who in the evening leave for distant homes.

The reason for all this seems on its face to be profanely simple: In the most economically vital region in the world, job growth has outpaced housing construction, making it harder for everyone to find affordable places to live.

There's a rub hidden in this simple formula for disaster, though, one that doesn't get bandied about much here: San Francisco has the room to build the housing the city needs, to slow spiraling rents and sales prices, to forestall manic sprawl, to restore sanity to a market wholly out of kilter.

But it won't.

If the mayor, supervisors, and citizens of San Francisco were to wake up one day and decide they wanted to confront the housing crisis, they could do so with relative ease. It would require nothing more than building the housing already permitted under existing laws and zoning plans, creating enough homes and apartments to house tens of thousands of people. According to the Planning Department, the city could build around 80,000 units -- or nine Brentwoods -- without changing current zoning laws one bit. And that's assuming most of the new housing would be built on vacant land.