Oct. 23--New and expanding companies filled gobs of new offices in the third quarter -- a remnant of the go-go deal-making that marked the Triangle only a year ago. But the activity belies what looms ahead for Triangle landlords, who have benefited from rampant demand and rising rents.

"The tables are turning," said Rich Harris, a broker at Synergy Commercial Advisors in Durham.

Net absorption, the total space leased minus the total vacated, was a whopping 632,000 square feet during the three months ending Sept. 30, according to data collected by Highwoods Properties, a Raleigh real estate investment trust. That's the most in any quarter since 2006.

But it wasn't enough to outpace a crush of new space hitting the market. New buildings totaling 754,000 square feet were completed during the quarter, of which 71 percent was leased.

As a result, the Triangle's office vacancy rate grew to a two-year high. So did anxiety among the builders who are racing to complete and fill 1.6 million square feet more.

The office vacancy rate climbed to 13.5 percent at the end of the third quarter, up from 12.4 percent a year ago. That's an important indicator for landlords and tenants, who pay close attention to the balance of supply and demand, in part to determine leverage in lease negotiations.

Triangle office landlords have had the upper hand for the past three years. As tenants fought for the best available spaces for their expansions, landlords responded by raising rents and offering few concessions.

But as the economy has soured in recent months, new and expanding tenants have become scarce. Some are downsizing. And those who want to lease are proceeding with caution, if at all.

"The next six to 12 months is going to be very difficult with respect to negotiations," Harris said, "because expectations are going to be dramatically different."

The quarter's strong leasing was heartening to developers. And while many hope for more of the same, few are betting on it. "It's not a leading indicator of the current mind-set and temperament of tenants making long- term commitments, which real estate falls into," said John Stubbs, a broker at Jones Lang LaSalle. "We're seeing a hesitancy to make a long-term commitment."

That caution is forcing some landlords to budge on rental rates or offer more interior construction dollars to seal the deal before tenants walk to a competitor or simply ditch expansion plans. Landlords are willing to give a little for the security of a rental stream.

That doesn't mean they're giving away the store.