“I think the story is for the first time in a while the major high-rise office buildings have space available,” said David Gurry, a Colliers senior vice president. “Certainly as rates fall we are starting to see a flight to quality.”

Tenants on lower floors are looking to move higher and those in Class B and C buildings can go to a higher class of space “for a comparable rate,” said Gurry. Additionally, some tenants in buildings outside the central business district are starting to look in the core.

But don't expect this flight to quality to lower vacancy near term, Gurry said, because businesses that are moving generally are just trading their space for the same square footage of higher quality space, and some are downsizing, he said. In general firms are not expanding downtown, he said.

“It's absolutely a shuffle right now,” he said. “It's activity I don't think is going to have an effect on vacancy either way. We're not in a market where tenants are growing.”

Colliers' Seattle Skyline Review gives a snapshot of vacancy in 10 prominent office buildings that are not newly built. It shows an additional 53,444 square feet of vacant space in the third quarter of this year compared to the second.

The large amount of new space coming on the market will cause vacancy to rise in the existing Class A buildings longer term, said Gurry.

According to Colliers data, the average rent for downtown Class A space in the first three months of this year is $31.04 per square foot, about $4.60 less per foot than the average for all of 2007, at the market's recent peak.

Dan Dahl, a senior vice president with the firm, said Colliers expects vacancy to continue to rise for the next couple of quarters, but he said that rents can only go so low.

“At a certain point landlords will just say ‘I'll wait'” rather than give away space too cheaply, he said.