SANTA CLARA — Silicon Valley’s commercial real estate boom should extend through the year, according to a closely watched forecast offered Wednesday by a Bay Area commercial realty firm.

“We are very excited and optimistic about 2013,” said Charles Seufferlein, CEO of Cornish & Carey Newmark Knight Frank, which presented the annual forecast in Santa Clara.

Developers are busy constructing 2.2 million square feet of office space in the South Bay on a speculative basis with no assurance of tenants for the projects, Cornish & Carey brokers said in their presentation.

“We haven’t seen that level of activity in about six, seven, eight years,” said Phil Mahoney, an executive vice president

The 680,000-square-foot building Samsung building will replace the 200,000 square feet of company offices now on the site at North First Street and Tasman Drive in San Jose. (Samsung)

with the brokerage.The square footage under construction is nearly equivalent to two regional shopping centers the size of Westfield Valley Fair. And more development may be coming.

“There is more in the pipeline that is being talked about,” Mahoney said. “Maybe four or five times that amount.”

Driving the commercial real estate speculation is the social networking and mobile revolution.

“This is smartphone and tablet valley,” Mahoney said.

In 2012, rents rose 5 percent in Class A offices, which are the top tier. Rents rose 17 percent for research and development space, Cornish & Carey reported during the presentation.

Vacancy levels for offices and research buildings combined were at 13.5 percent

at the end of 2012, down from 14.6 percent at the end of 2011.The expansion in Silicon Valley, the Peninsula and San Francisco goes beyond a few famous players.”It is not just Apple (AAPL), or Google (GOOG), or Facebook or LinkedIn,” Mahoney said.

Among the rapidly growing companies are many that most people have never hear of, including Box, Twitter, Arista Networks, Square, Violin Memory, Cloudera, Gigamon, iWatt, Xoom, Survey Monkey, Kabam, EventBrite, Zendesk, Marin Software, Palatir, Sugar CRM, Nimble Storage and Marketo.

Erik Doyle, executive managing director of Cornish & Carey, estimated that the Bay Area is now “somewhere in the sixth inning” of its current commercial real estate boom cycle.

But some experts are already cautioning that developers could launch too many buildings, and wind up stuck with a glut of offices on their hands.

In 2008, developers planned to construct 24 million square feet of offices in Silicon Valley. Then the recession savaged the economy and only 2 million square feet were built, said Jim Beeger, a senior vice president with Colliers International, a commercial realty firm.

“We never do things in moderation here,” Beeger said. “We don’t just super-size it. We double-super-size it.”

Contact George Avalos at 408-373-3556 or 925-977-8477. Follow him at Twitter.com/george_avalos.