Moving out of Silicon Valley to find affordable housing in California now means moving further than ever as the half-million dollar median home price spreads throughout the San Francisco Bay Area.

Back in 2000, only a few counties in Northern California—including Santa Clara—yielded a half-million dollar median priced home and then only for single-family detached homes.

In just four years, the fear of rising interest rates and the region's continued desirability is sending price ripples throughout the greater nine-county San Francisco Bay Area housing market.

Now, for all homes in the region—single-family detached and condos, both new and resale—the median price peaked at a record $506,000 in May, according to La Jolla-based DataQuick Information Systems, which tracks prices and other statistics on closed transactions on all homes in California and elsewhere.

By county, only four—Alameda, Contra Costa, Solano and Sonoma—still have median home prices below $500,000, with Alameda and Contra Costa likely to reach the $500,000 mark before year's end should pricing trends continue.

Five counties—Marin, Napa, San Francisco, San Mateo and Santa Clara—all have median prices higher than $500,000, with Marin, San Francisco and San Mateo counties yielding medians higher than $600,000.

There's little, if any, long-term relief in sight.

"I never bought into that bubble theory," said Richard Calhoun, San Jose broker/owner of Creekside Realty and publisher of the Bay Area Real Estate Market Newsletter, which tracks real estate statistics only on resale homes in a five-county area served by the regional multiple listing service, RE InfoLink.

In Santa Clara County, the median price for only single-family detached homes stands at a record $635,000 as of May according to Calhoun.

"Prices will continue to go up in the long run, barring some unforeseen circumstance. I don't know why people think there has to be a limit. Look at the price of gasoline. It used to be 29.9 cents a gallon, and milk, what was that, a buck a gallon?" he said.

Dataquick's median price reflects an 18.5 percent jump from May last year—the largest year-over-year increase since January 2001.

Prices in the region are being driven up by the growing number of sales.

For all homes, a total of 12,028 were sold in the region in May, up 13.4 percent from May last year, according to DataQuick. In Santa Clara County sales rose 30.7 percent during the same period.

Increased sales are the result of both current market demand and the persistent desire to live in the region for it's climate, topography and diversity.

Sales are also pushed by buyers rushing to stay one step ahead of rising interest rates which have already risen a full percentage point this year in anticipation of federal benchmark interest rate hikes. As long as low rates allow buyers to afford the monthly mortgage payment, rising prices won't stall sales.

"We are surrounded by water and mountains and a limited supply of land, but it is a very nice place to live. It's the weather and the opportunity and just the lure of being in Silicon Valley," said Calhoun.

John Karevoll, spokesman for DataQuick, says the market is also driven by demand factors found in any market.

"People get married, they have kids, they get new jobs, they get fired from old jobs," Karevoll said.

The demand feeds upon itself as home buyers realize, in addition to shelter in a good location, they've also landed a solid investment.

"It's always a long-term investment. In 1981, my dad thought I was crazy buying a house in Campbell that was double the price of a house and in an inferior neighborhood. When people own real estate, they drop anchor and keep doing it," said Calhoun.

Some short-term relief on price pressure may appear as more homes are listed for sale and the major buying seasons wanes later this year.

"There are indications now that more homes are being put up for sale. That should take some of the upward pressure off prices. It also means that sales counts will stay strong throughout the summer," said Marshall Prentice, DataQuick president.

Real estate writer Broderick Perkins, executive editor of San Jose-based DeadlineNews.Com, writes regularly for this newspaper.