SAN RAFAEL -- The Marin County Board of Supervisors today approved an $81.9 million plan to purchase a 328,000-square-foot mostly vacant office building and renovate up to 110,000 square feet to accommodate a new county Emergency Operations Facility.

The action allows county staff to execute an agreement with building owner CORAC, LLC, an affiliate of Connecticut-based Cigna Real Estate, to purchase 1600 Los Gamos Dr. for $28.43 million, according to documents presented to the board. The sale is set to close next month.

Then the county intends to spend another $53.1 million through early to mid-2014 on improvements to the building and nearly 24-acre parcel, furniture, fixtures and equipment. The facility will accommodate the Sheriff's, Information Services and Technology and Public Works departments in 100,000 to 110,000 square feet of the vacant north part of the building.

Three major tenants -- Bright Horizons, the IRS and THX -- will continue to occupy about 55,000 square feet of the building. Cigna will continue to manage the building through the end of 2014.

Over the next three years, the county plans to use 80,000 square feet of vacant space for temporary operations, such storage for the Registrar of Voters and during renovation of 20 N. Pedro Rd., which houses a number of Health & Human Services Department offices.

That will leave 63,000 square feet of office space and 20,000 square feet of storage space to lease.

Local commercial property experts have been eagerly anticipating the county's action on Marin Commons, one of the largest office properties in the county. The space has inflated the Marin office vacancy rate, at 18.1 percent in the third quarter, according to Cassidy Turley BT Commercial. Of the 1.78 million square feet of available office space in the county, 986,000 square feet are in San Rafael, mostly in a few large-tenant properties.

Marin Commons was designed for one or a few large tenants, making it functionally obsolete for most of the tenants scouting for Marin offices, according to real estate agents.

"North San Rafael been slow for several years," said Trevor Buck of Cassidy Turley BT Commercial.

For example, Healthnet vacated a large portion of 1600 Los Gamos, and the hilltop 1 Thorndale building lost Disney's ImageMovers Digital and a video-game maker now called 2K Marin to Novato, though ImageMovers Digital left Marin soon thereafter.

"The submarket generally had a paper vacancy rate, but the vacancy rates will drop considerably," Mr. Buck said.

Those two buildings and some other significant vacancies in north San Rafael have been filling up in the past year.

The 1600 Los Gamos building is the largest of the two-building Marin Commons office complex, located in north San Rafael. It was finished in 1973 and last renovated in 1995.  Not included in the sale is the 141,000-square-foot 1650 Los Gamos building, built in 1979 and renovated in 2007 with new lobbies and some ready-to-occupy suites.

Cigna foreclosed on the property in 2009 and brought in Sares Regis Group of Northern California to reposition the property early the following year. Cigna and the county were in talks about a significant occupancy in the complex for more than a year.

The county had considered the purchase of the south and middle portions of the building, totaling 165,000 square feet. Last month, the board of supervisors decided to move forward with the full-building purchase after staff analysis found that a lot split, installation of a firewall and other upgrades needed to purchase just a portion would cost $2.2 million to $7.4 million more, according to a staff report.