The next six to nine months will be a challenge for Los Angeles businesses that have to rejigger their budgets to afford higher rents or scout out new space for expansion.

That’s the projection from Grubb & Ellis Co., which tracks the region’s office and industrial space and leasing rates.

The countywide vacancy rate at the end of the second quarter not only continued to hover in the high single digits 9.5 percent for the third consecutive quarter but lease rates for Class A office space have soared nearly 15 percent in the past year alone.

Landlords are asking an average $3.10 per square foot for Class A space, with rents in the most desirable Westside communities fast approaching the $6 mark. Even shoppers for Class B space in the county’s oldest commercial neighborhoods are seeing asking rents 8 percent higher than a year ago.

“The second half of the year is going to be very interesting,” is how J.C. Casillas, Grubb’s client services manager, puts it delicately.

Lease rates, largely driven by lack of inventory, are not yet to the point where they’re driving businesses out of the county, but some say they’re fast approaching that state.

“We’re at historically low vacancies rates, and that’s going to continue because there’s been very little development,” said Joe Vargas, managing director for Cushman & Wakefield’s Southern California office. “On top of classic supply and demand you have investors coming in and buying at a high square foot price in both office and industrial buildings and they need to get a return on that.”

More than 2 million square feet of office construction is underway around the county, but that’s a drop in the bucket considering that L.A. County is nearly a 186 million square-foot market.

Casillas points to a key statistic, net absorption rates, as an indicator of how tight the market is. Net absorption is a measure of how much space is taken off the market through leasing. It can go negative when more tenants leave than lease or construction dumps new space on the market.

Net absorption countywide in the quarter stood at 109,961 square feet 93 percent less than the same period in 2006, an indicator of how little space there was available.

Indeed, in some extremely tight submarkets which continue to be highly desirable for prestige or affordability reasons such as Century City, Westwood, El Segundo, and the West San Fernando Valley were among nearly a dozen areas that oddly saw negative net absorption in the second quarter.

“There’s a little bit of musical chairs happening as tenants move out, temporarily leaving a hole for a quarter until the next guy to move in,” Casillas said. “It’s been happening in the industrial market for some time, but we’re now seeing several office markets, most with single digit vacancy rates, that have these significant shifts from quarter to quarter.”

Consider Century City. Law firms, architectural-engineering companies and other professional services that have had enough of the area’s high lease rates are filling up long neglected space downtown and elsewhere. Insurance broker C.V. Star & Co. was among tenants relocating to a revitalized downtown, moving into the top floor of 1000 Wilshire Blvd. during the quarter.

“It’s a trend I see continuing in the second half of the year,” said John McAniff, managing director at Jones Lang LaSalle, adding that El Segundo and the surrounding beach cities, with rents under $3, also will see a lot of absorption going forward from relocating tenants.

Meanwhile, the higher demand for downtown space has pushed up rents there a modest 3.5 percent for Class A space and 3 percent for Class B. McAniff sees a further gradual climb in rents, but the bigger issue is that there’s a shrinking number of large blocks of space for expansion.

While a year ago there were around seven different places at City National Plaza where a broker could put two floors of consecutive unencumbered space together for a tenant, he estimates that there may be only one block like that left today.

“If you need 50,000 to 70,000 square feet downtown, you have less than half the options you had a year ago,” he said. “There’s also a lot less space available for sublease, which otherwise would hold down rents.”

The countywide industrial vacancy rate continues to rest at the practically full 1.8 percent it stood at a year ago, but net absorption there plunged 14 percent, largely from vacancies in the Mid-Cities area. Rents returned to the 60 cents per square foot rate they were a year ago, after briefly rising to 61 cents in the first quarter.