The vacancy rate for O'ahu office space rose slightly in the first quarter, the first increase in nearly four years, according to a new report.

But report author Colliers Monroe Friedlander said it still expects occupancy to increase for the full year as the economy continues to expand.

The local commercial real estate firm reported that 4,148 square feet more office space became vacant on O'ahu during the first three months of the year, pushing the vacancy rate to 7.17 percent from 7 percent at the end of last year.

The uptick in vacancy follows an almost four-year decrease in vacancies that fell from about 14 percent in mid-2003 when the market experienced its highest vacancy rate in more than a decade.

Mike Hamasu, Colliers director of research and consulting, said the first-quarter drop was small and should be followed by 100,000 to 150,000 square feet of office space absorption by year-end.

"I don't see it as a Chicken Little thing where the sky is falling," he said. "It's more of a lateral move. Really, (4,148 square feet) is one tenant."

Hamasu said Class A buildings, the priciest, lost 31,569 square feet of occupancy, primarily because two law firms made significant space available for sublease after partners left to form their own firms.

Lower-tier Class B and C buildings had 27,000 square feet of empty space filled. Part of the reason cheaper buildings are filling up is that rising rents have driven some businesses to seek less expensive digs, generally older buildings outside Downtown.

"That's been a trend over the last couple of years," Hamasu said.

The average monthly asking rent during the first quarter was $2.65 a square foot, up from $2.59 at the end of last year. In mid-2003, the average rate was $2.12.

Reach Andrew Gomes at agomes@honoluluadvertiser.com.